Magnetoelastic torque transducer

ABSTRACT

A magnetoelastic torque transducer for providing an electrical signal indicative of the torque applied to a member, the member including ferromagnetic, magnetostrictive means affixed to, associated with or forming a part of the surface of the torqued member for altering in magnetic permeability in response to the application of torque to the member. The ferromagnetic, magnetostrictive means is advantageously formed of a thermally hardened or iron-nickel martensite hardenable steel alloy characterized by a substantially isotropic magnetostriction having an absolute value of at least 5 ppm and including from 0.05 to 0.75 percent by weight carbon and sufficient of one or more elements selected from Ni, Cr, Co, Ti, Al, Mn, Mo, Cu and B to raise the alloy magnetostriction to at least 5 ppm absolute. Preferably, the ferromagnetic, magnetostrictive means is formed of nickel maraging steel. The transducer comprises a pair of axially spaced-apart annular bands defined within a region of the ferromagnetic, magnetostrictive means, the bands being endowed with residual stress created, respectively symmetrical right and left hand helically directed magnetic anisotropy of sufficiently large magnitude that the contribution to total magnetic anisotropy of any random anisotropy in the member is negligible. In one aspect of the invention, each said band has at least one circumferential region which is free of residually unstressed areas over at least 50% of its circumferential length. In another aspect of the invention, the alloy is thermally hardened before the bands are endowed with the residual stress-created magnetic anisotropy.

Cross-Reference To Related Applications

This application is a continuation-in-part of copending U.S. applicationSerial No. 938,404 filed Dec. 5, 1986 , now U.S. Pat. No. 4,760,745 andcopending U.S. Pat. application Ser. No. 095,774 filed Sept. 14, 1987.

Technical Field

The present invention relates to torque sensors and, more particularly,to non-contacting magnetoelastic torque transducers for providing ameasure of the torque applied to a rotary shaft.

Background Art

In the control of systems having rotating drive shafts, it is generallyrecognized that torque is a fundamental parameter of interest.Therefore, the sensing and measurement of torque in an accurate,reliable and inexpensive manner has been a primary objective of workersfor several decades. Although great strides have been made, thereremains a compelling need for inexpensive torque sensing devices whichare capable of continuous torque measurements over extended periods oftime despite severe environments.

All magnetoelastic torque transducers have two features in common--(1) atorqued member which is ferromagnetic and magnetostrictive, the formerto ensure the existence of magnetic domains and the latter to allow theorientation of the magnetization within each domain to be altered by thestress associated with applied torque; and (2) a means, most usually butnot necessarily electromagnetic means, for sensing variations from theuntorqued distribution of domain orientations. The differences among thevarious existing or proposed magnetoelastic torque transducers lie inthe detailed variations of these common features.

It is well known that the permeability of magnetic materials changes dueto applied stress. When a torsional stress is applied to a cylindricalshaft of magnetostrictive material, each element in the shaft issubjected to a shearing stress. This shearing stress may be expressed interms of a tensile stress and an equal and perpendicular compressivestress, with the magnitude of each stress being directly proportional tothe distance between the shaft axis and the element. The directions ofmaximum tension and compression occur along tangents to 45° left-handedand 45° right handed helices about the axis of the shaft. The effect ofthe torque is to increase the magnetic permeability in directionsparallel to one of the helices and, correspondingly, to decrease themagnetic permeability in directions parallel to the other of thehelices. In their article "Magnetic Measurements of Torque in a RotatingShaft", The Review of Scientific Instruments, Vol. 25, No. 6, June,1954, Beth and Meeks suggest that in order to use permeability change asa measure of the applied torque, one should monitor permeability alongthe principal stress directions and pass the magnetic flux through theshaft near its surface. This is because the stress is greater, thefurther the element is from the shaft axis and it is along the principalstress directions that the maximum permeability change is expected. Toaccomplish this, Beth and Meeks used a yoke carrying a driving coil forproducing an alternating flux in the shaft and pickup coils on each ofseveral branches to detect the permeability changes caused by theapplied torque in flux paths lying in or near the principal stressdirections in the shaft. When the shaft is subjected to a torque, themechanical stresses attributable to torque resolve into mutuallyperpendicular compressive and tensile stresses which cause thepermeability in the shaft to increase in the direction of one stress anddecrease in the direction of the other. As a result, the voltage inducedin the pickup or measuring coils increases or decreases. The differencein magnitude of the induced voltages is proportional to the torsionalstress applied to the shaft. A similar approach was taken in U.S. Pat.No. 3,011,340--Dahle. The principal shortcoming in these type devices isthe need to accomplish permeability sensing along the principal stressdirections with its attendant disadvantages, such as its sensitivity tovariations in radial distance from the shaft, magnetic inhomogeneityaround the shaft circumference and noncompensatable dependence on shaftspeed. As a result, devices such as these have only found applicationson large diameter shafts, i.e., 6-inches and larger, but have not beenfound to be adaptable to smaller shafts where the vast majority ofapplications exist.

It was felt by some that devices such as were taught in Beth and Meeksand U.S. Pat. No. 3,011,340 --Dahle, wherein the rotating shaft itselfacted as the magnetic element in the transducer, had significantdrawbacks in practical application. This is because the materials andmetallurgical processing which may have been used to impart the desiredmechanical properties to the shaft for its desired field of use will, inmost cases, not be optimum or even desirable for the magnetic qualitiesrequired in a magnetoelastic torque sensor. The random anisotropy in ashaft created during its manufacture, due to internal stresses and/orresulting from regions of differing crystal orientation, will causelocalized variations in the magnetic permeability of the shaft whichwill distort the desired correlation between voltage sensed and appliedtorque. The solution, according to U.S. Pat. No. 3,340,729--Scoppe is torigidly affix, as by welding, a magnetic sleeve to the load-carryingshaft so that a torsional strain proportional to the torsional load isimparted to the sleeve. The measuring device employed now sensespermeability changes in the rotating sleeve rather than in the rotatingshaft. This permits, according to Scoppe, a material to be selected forthe shaft which optimizes the mechanical and strength propertiesrequired for the shaft while a different material may be selected forthe sleeve which optimizes its magnetic properties. As with prior artdevices, the Scoppe torquemeter utilized a primary winding forgenerating a magnetic flux and two secondary windings, one oriented inthe tension direction and the other in the compression direction.Although obviating at least some of the materials problems presented byDahle, the use of a rigidly affixed sleeve creates other, equallyperplexing problems. For example, the task of fabricating and attachingthe sleeve is a formidable one and even when the attachment means iswelding, which eliminates the bond strength problem, there remains thevery significant problem that the coefficient of thermal expansion ofthe steel shaft is different (in some cases up to as much as 50%greater) than the corresponding coefficient of any magnetic materialselected for the sleeve. A high temperature affixing process, such aswelding, followed by cooling establishes stresses in the magneticmaterial which alters the resultant magnetic anisotropy in anuncontrolled manner. Moreover, annealing the shaft and sleeve to removethese stresses also anneals away desirable mechanical properties in theshaft and changes the magnetic properties of the sleeve. Furthermore,like the Dahle device, the shortcomings of Scoppe's transducer, due toits need to monitor permeability changes lying along the principalstress directions, are its sensitivity to variations in its radialdistance from the shaft, magnetic inhomogeneity around the shaftcircumference and dependence on shaft speed.

A different approach to magnetoelastic torque sensing utilizes thedifferential magnetic response of two sets of amorphous magnetoelasticelements adhesively attached to the torqued shaft. This approach has theadvantage over prior approaches that it is insensitive to rotationalposition and shaft speed. However, it requires inordinate care in thepreparation and attachment of the elements. Moreover, transducerperformance is adversely affected by the methods used to conform theribbon elements to the shape of the torqued member; the properties ofthe adhesive, e.g., shrinkage during cure, expansion coefficient, creepwith time and temperature under sustained load; and, the functionalproperties of the amorphous material itself, e.g., consistency,stability. Still another concern is in the compatibility of the adhesivewith the environment in which the transducer is to function, e.g., theeffect of oil, water, or other solvents or lubricants on the propertiesof the adhesive.

In the article "A New Torque Transducer Using Stress Sensitive AmorphousRibbons", IEEE Trans. on Mag., MAG-18, No. 6, 1767-9, 1982, Harada etal. disclose a torque transducer formed by gluing two circumferentialstress-sensitive amorphous ribbons to a shaft at axially spaced apartlocations. Unidirectional magnetoelastic magnetic anisotropy is createdin each ribbon by torquing the shaft in a first direction, gluing afirst ribbon to it, releasing the torque to set-up elastic torquestresses within the first ribbon, torquing the shaft in the oppositedirection, gluing the second ribbon to it, and then releasing the torqueto set-up elastic torque stresses within the second ribbon. The resultis that the anisotropy in one ribbon lies along a right-hand helix at+45° to the shaft axis while the anisotropy in the other ribbon liesalong an axially symmetric left-hand helix at -45° to the shaft axis. ACpowered excitation coils and sensing coils surround the shaft making thetransducer circularly symmetric and inherently free from fluctuation inoutput signal due to rotation of the shaft. In the absence of torque,the magnetization within the two ribbons will respond symmetrically toequal axial magnetizing forces and the sensing coils will detect nodifference in the response of the ribbons. However, when torque isapplied, the resulting stress anisotropy along the principal axesarising from the torque combines asymmetrically with the quiescentanisotropies previously created in the ribbons and there is then adiffering response of the two ribbons to equal axial magnetizing force.This differential response is a function of the torque and the sensingcoils and associated circuitry provide an output signal which isproportional to the applied torque. Utilizing substantially the sameapproach, in Japanese patent publication No. 58-9034, two amorphousribbons are glued to a shaft and symmetrical magnetic anisotropy isgiven to the ribbons by heat treatment in a magnetic field atpredetermined equal and opposite angles. Amorphous ribbons have alsobeen glued to a shaft in a ±45° chevron pattern, see Sasada et al., IEEETrans. on Mag., MAG-20, No. 5, 951-53, 1984, and amorphous ribbonscontaining parallel slits aligned with the ±45° directions have beenglued to a shaft, see, Mohri, IEEE Trans. on Mag., MAG-20, No. 5,942-47, 1984, to create shape magnetic anisotropy in the ribbons ratherthan magnetic anisotropy due to residual stresses. Other recentdevelopments relevant to the use of adhesively attached amorphousribbons in a magnetoelastic torque transducer are disclosed in U.S. Pat.No. 4,414,855--Iwasaki and U.S. Pat. No. 4,598,595--Vranish et al.

More recently, in apparent recognition of the severe shortcomingsinherent in using adhesively affixed ribbons, plasma spraying andelectrodeposition of metals over appropriate masking have been utilized.See: Yamasaki et al, "Torque Sensors Using Wire ExplosionMagnetostrictive Alloy Layers", IEEE Trans. on Mag., MAG-22, No. 5,403-405 (1986); Sasada et al, "Noncontact Torque Sensors Using MagneticHeads and Magnetostrictive Layer on the Shaft Surface Application ofPlasma Jet Spraying Process", IEEE Trans. on Mag., MAG-22, No. 5,406-408 (1986).

The hereinbefore described work with amorphous ribbons was not the firstappreciation that axially spaced-apart circumferential bands endowedwith symmetrical, helically directed magnetic anisotropy contributed toan improved torque transducer. USSR Certificate No. 274,420 discloses amagnetoelastic torque measuring device, not unlike the Harada et alamorphous ribbon transducer, comprising a pair of sleeves which areinitially deformed by applied torques of different directions to endowthe sleeves with oppositely directed magnetic anisotropy and thenmounted on a shaft in annular grooves formed therein. The grooves have aradial depth selected to accommodate the sleeves therewithin with theouter diameter of the sleeves coplanar with the outer surface of theshaft. Appropriate electronic circuitry is employed to sense thepermeability change in the sleeves when a torque is applied to the shaftand to produce a corresponding electrical signal. There is no indicationof the materials employed for the sleeves or the shaft. There is also nodisclosure regarding the manner in which the deformed sleeves areaffixed to the shaft. Whatever the technique, adhesive or welding, theresulting torque measuring device will suffer from the same drawbacks aswith Scoppe's welded sleeves (U.S. Pat. No. 3,340,729) or Harada'sadhesively affixed ribbons. USSR Certificate No. 667,836 discloses amagnetoelastic torque transducer having two axially spaced-apartcircumferential bands on a shaft, the bands being defined by a pluralityof slots formed in the shaft in a ±45° chevron pattern, and a pair ofexcitation and measuring coil-mounting circumferential bobbins axiallylocated along the shaft so that a band underlies each bobbin. The shapeanisotropy created by the slots is the same type of magneticpreconditioning of the shaft as was created, for example, by thechevron-patterned amorphous ribbons of Sasada et al and the slittedamorphous ribbons of Mohri, and suffers from many of the sameshortcomings. Japanese Pat. No. 169,326 discloses means for measuringthe torque in a rotating shaft formed of ferromagnetic material. Themeans includes a pair of axially spaced-apart bands on the shaftsurface, the bands having knurls formed thereon at opposite angles of ±45° and coils surrounding the respective bands for sensing the change inmagnetic flux when torque is applied to the shaft and for generating anemf proportional to the applied torque. There is no teaching in thepatent of the process used for applying the knurl, of the ferromagneticmaterial used for the band portions of the shaft or of any thermaltreatments of the shaft to anneal away stresses or to impart mechanicalstrength. Moreover, there is no disclosure in the patent of the specificknurl configuration or trough density, although the drawings suggestthat the troughs are widely spaced apart. Certainly, there is noteaching that the bands include at least one circumferential regionwhich is free of residually unstressed areas over at least 50% of itscircumferential length. Moreover, the reference to the knurl "hills" inone band being subjected to a compressive stress while the knurl "hills"in the other band are subjected to a tensile stress suggest that themagnetic anisotropy results from the macroscopic topographic alterationof the shaft surface, i.e., the knurl, rather than from any residualstress created by a mechanical working process leading to the knurl.Thus, the patent appears to teach that magnetic anisotropy resultingfrom topographic alteration, rather than residual stress createdmagnetic anisotropy, is responsible for the sensed change inpermeability. This teaching is entirely consistent with the formation ofa knurl having relatively widely spaced apart troughs by a technique,such as machining or photoetching, which imparts no residual stresscreated anisotropy to the bands. A torque measuring device exclusivelyor substantially dependent upon topographic alteration possesses toolittle anisotropic preconditioning in the bands to provide a practicallyuseful sensitivity. USSR Certificate No. 838,448 also discloses amagnetoelastic torque transducer having two spaced-apart circumferentialbands on a shaft, circumferential excitation coils and circumferentialmeasuring coils surrounding and overlying the bands. In this transducerthe bands are formed by creating a knurl in the shaft surface with thetroughs of the knurl at ±45° angles to the shaft axis so that thetroughs in one band are orthogonal to the troughs in the other band. Theknurls are carefully formed by an undisclosed method which ensures thepresence of substantial unstressed surface sections between adjacenttroughs so that the magnetic permeability of the troughs is differentfrom the magnetic permeability of the unstressed areas therebetween.Inasmuch as the trough width-to-pitch ratio corresponds to the stressedto unstressed area ratio and the desired ratio appears to be 0.3, thereis no circumferential region in either band which is intentionallystressed over more than 30% of its circumferential length. This veryminimal stress anisotropic preconditioning is believed to be too smallto provide a consistent transducer sensitivity, as measured by theelectronic signal output of the measuring coils and their associatedcircuitry, for economical commercial utilization.

Notwithstanding their many shortcomings in forming sensitive andpractical bands of magnetic anisotropy on a torqued shaft, the effortsevidenced in the Harada et al, Sasada et al, Mohri and Yamasaki et alarticles and the USSR certificates represent significant advances overthe earlier work of Beth and Meeks, Dahle and Scoppe in recognizing thata pair of axially spaced-apart, circumferential bands of symmetrical,helically directed anisotropy permits averaging axial permeabilitydifferences over the entire circumferential surface. This is notablysimpler than attempting to average helical permeability differencessensed along the principal stress axis, as had earlier been suggested.Moreover, neither rotational velocity nor radial eccentricitysignificantly influence the permeability sensed in this manner.Nevertheless, these efforts to perfect means of attachment ofmagnetoelastically optimized material to the surface of the torquedmember introduces unacceptable limitations in the resulting torquesensor. The application to the shaft of adhesively affixed amorphousribbons suffers from significant drawbacks, such as the methods used toconform the ribbons to the shaft, the properties of the adhesive and thefunctional properties of the amorphous material, which make such ribbonsimpractical for commercial implementation. The use of rigidly affixedsleeves as taught by Scoppe and, more recently, in U.S. Pat. No.4,506,554--Blomkvist et al, is unsuitable for practical applications dueto the higher costs involved as well as the stresses created by hightemperature welding and/or the uncertainties in magnetic and mechanicalproperties created by subsequent annealing. Likewise, reliance uponshape anisotropy or predominantly unstressed regions to create stressanisotropy present significant problems which make such techniquesimpractical for commercial implementation.

It is, therefore, apparent that despite the many advances in torquetransducer technology, there still exists a need for a magnetoelastictorque transducer which is significantly more economical than previoustorque transducers, allowing use in many applications for which suchtransducers were not heretofore either economically or environmentallyviable, and which is applicable to small as well as large diametershafts, whether stationary or rotating at any practical speed.

Disclosure of the Invention

In accordance with one broad aspect of the present invention there isprovided a magnetoelastic torque transducer for providing an electricalsignal indicative of the torque applied to a member in which aferromagnetic and magnetostrictive region of the torqued m ember servesas a part of the magnetic sensing circuit of the transducer by providingat the surface of said region a pair of axially spaced-apart annularbands endowed with residual stress created, respectively symmetrical,left and right hand helically directed magnetic anisotropy of relativelylarge magnitude, which anisotropy overwhelms and/or renders negligibleor insignificant any random anisotropy in the member as a result ofinternal stresses due to mechanical working, inhomogeneities, crystalorientation, and the like.

In another aspect of the present invention there is provided amagnetoelastic torque transducer for providing an electrical signalindicative of the torque applied to a member in which a thermallyhardened ferromagnetic, magnetostrictive region of the torqued member ora thermally hardened ferromagnetic, magnetostrictive means rigidlyaffixed to or associated with the surface of said torqued member servesas a part of the magnetic sensing circuit of the transducer by providingat the surface of said thermally hardened region or means a pair ofaxially spaced-apart annular bands endowed with residual stress created,respectively symmetrical, left and right hand helically directedmagnetic anisotropy of relatively large magnitude, which anisotropyoverwhelms and/or renders negligible or insignificant any randomanisotropy in the member as a result of internal stresses due tomechanical working, inhomogeneties, crystal orientation, and the like.As used herein, the term "thermally hardened" means heat treated atelevated temperatures above about 800° C., e.g., by annealing followedby quenching or by case hardening at high temperatures in a carburizingatmosphere followed by quenching, to impart desirable mechanicalproperties, e.g., hardness and strength, to the material of which themember, region or means is formed.

In accordance with another aspect of the present invention, there isprovided a magnetoelastic torque transducer for providing an electricalsignal indicative of the torque applied to a member, said member havinga ferromagnetic and magnetostrictive region, said transducer comprisinga pair of axially spaced-apart annular bands defined within said region,said bands having, at least at the surface of said member, respectivelysymmetrical right and left hand helically directed residual stresscreated magnetic anisotropy, each said band having at least onecircumferential region which is free of residually unstressed areas,i.e., said at least one circumferential region is residually stressed,over at least 50% of its circumferential length; means for applying acyclically time varying, e.g., alternating. magnetic field to saidbands; means for sensing the change in permeability of said bands causedby said applied torque; and means for converting said sensed change inpermeability to an electrical signal indicative of the magnitude of thetorque applied to said member. In a preferred aspect, the ferromagneticand magnetostrictive region is formed of an iron-nickel martensitehardenable steel, such as a nickel maraging steel, or a thermallyhardened steel alloy characterized by a substantially isotropicmagnetostriction having an absolute value of at least 5 ppm andincluding from 0.05 to 0.75 percent by weight carbon and sufficient ofone or more elements selected from Ni, Cr, Co, Ti, Al, Mn, Mo, Cu and Bto raise the alloy magnetostriction to at least 5 ppm absolute.

In accordance with another aspect, the present invention contemplates amagnetoelastic torque transducer for providing an electrical signalindicative of the torque applied to a member, including ferromagnetic,magnetostrictive means rigidly affixed to, associated with or forming apart of the surface of said torqued m ember for altering in magneticpermeability in response to the application of torque to said member,means for applying a magnetic field to said ferromagnetic,magnetostrictive means, means for sensing the change in permeabilitycaused by said applied torque and means for converting said sensedchange in permeability to an electrical signal indicative of themagnitude of the torque applied to said member, the ferromagnetic,magnetostrictive means being formed of an iron-nickel martensitehardenable steel, such as a nickel maraging steel, or a thermallyhardened steel alloy characterized by a substantially isotropicmagnetostriction having an absolute value of at least 5 ppm andincluding from 0.05 to 0.75 percent by weight carbon and sufficient ofone or more elements selected from Ni, Cr, Co., Ti, Al, Mn, Mo, Cu and Bto raise the alloy magnetostriction to at least 5 ppm absolute.

In still another aspect of the present invention, there is provided amethod of sensing the torque applied to a member having a ferromagneticand magnetostrictive region, which includes the steps of endowing a pairof axially spaced-apart annular bands within said region withrespectively symmetrical, right and left hand helically directedmagnetic anisotropy, applying a cyclically time varying, e.g.,alternating, magnetic field to said bands and sensing the permeabilitydifference between said bands resulting from the application of torqueto said member, the difference being indicative of the magnitude of theapplied torque, the improvement which comprises forming said bands atthe surface of said member and endowing said bands with magneticanisotropy by instilling a residual stress distribution in each bandwhich is sufficiently extensive that at least one circumferential regionwithin each band is free of residually unstressed areas, i.e., said atleast one circumferential region is residually stressed, over at least50% of its circumferential length. In a preferred aspect of this method,the ferromagnetic and magnetostrictive region is formed of aniron-nickel martensite hardenable steel, such as a nickel maragingsteel, or a thermally hardened steel alloy characterized by asubstantially isotropic magnetostriction having an absolute value of atleast 5 ppm and including from 0.05 to 0.75 percent by weight carbon andsufficient of one or more elements selected from Ni, Cr, Co., Ti, Al,Mn, Mo, Cu and B to raise the alloy magnetostriction to at least 5 ppmabsolute.

In yet another aspect of the invention, there is provided a method ofsensing the torque applied to a member having a ferromagnetic andmagnetostrictive region which includes the steps of endowing said regionwith helically directed magnetic anisotropy by instilling a residualstress distribution in said region which is sufficiently extensive thatat least one circumferential region within said ferromagnetic andmagnetostrictive region is free of residually unstressed areas, i.e.,said at least one circumferential region is residually stressed, over atleast 50% of its circumferential length, applying a cyclically timevarying, e.g., alternating, magnetic field to said ferromagnetic andmagnetostrictive region and to an area of said member not so endowed,and sensing the permeability difference between said ferromagnetic andmagnetostrictive region and said area resulting from the application oftorque to said member, the difference being indicative of the magnitudeof the applied torque. In a preferred aspect of this method, theferromagnetic and magnetostrictive region is formed of an iron-nickelmartensite hardenable steel, such as a nickel maraging steel, or athermally hardened steel alloy characterized by a substantiallyisotropic magnetostriction having an absolute value of at least 5 ppmand including from 0.05 to 0.75 percent by weight carbon and sufficientof one or more elements selected from Ni, Cr, Co, Ti, Al, Mn, Mo, Cu andB to raise the alloy magnetostriction to at least 5 ppm absolute.

In still another aspect of the invention, there is provided a method ofsensing the torque applied to a member having a ferromagnetic andmagnetostrictive region in which the ferromagnetic and magnetostrictiveregion is thermally hardened prior to endowing said region withhelically directed magnetic anisotropy.

In a further aspect of the present invention, there is provided a methodfor making a magnetoelastic torque transducer for providing anelectrical signal indicative of the torque applied to a membercomprising the steps of providing a member having a ferromagnetic andmagnetostrictive region; thermally hardening said region; endowing saidthermally hardened region with a pair of axially spaced-apart annularbands having respectively symmetrical, right and left hand helicallydirected residual stress created magnetic anisotropy, each said bandhaving at least one circumferential region which is free of residuallyunstressed areas over at least 50% of its circumferential length;providing excitation means associated with said bands for applying acyclically time varying, e.g., alternating, magnetic field to saidbands; providing sensing means associated with said bands for sensingthe change in permeability of said bands caused by said applied torque;and, providing means for converting said sensed change in permeabilityto an electrical signal indicative of the magnitude of the torqueapplied to said member.

Brief Description of the Drawings

The invention will be better understood from the following descriptiontaken in conjunction with the accompanying drawings in which:

FIG. 1 is a perspective view of a magnetoelastic torque transducer inaccordance with the present invention;

FIG. 2 is a sectional view of a magnetoelastic torque transducer inaccordance with the present invention illustrating one form of magneticdiscriminator useful therewith;

FIG. 3 is a circuit diagram showing the circuitry associated with themagnetic discriminator of FIG. 2;

FIG. 4 is a schematic view of a magnetoelastic torque transducer inaccordance with the present invention illustrating another form ofmagnetic discriminator, and its associated circuitry, useful therewith;

FIG. 5 is a graphical representation of the relationship between appliedtorque and output signal for several magnetoelastic torque transducersof the present invention;

FIG. 6 is a graphical representation of the relationship between appliedtorque and output signal for the magnetoelastic torque transducers ofFIG. 5 after the shafts thereof have been heat treated under identicalconditions;

FIG. 7 is a graphical representation of the general relationship betweentorque transducer sensitivity and residual stress loading along thecircumferential length of a circumferential region of the bands of atransducer of the present invention;

FIG. 8 is an elevational view of a test piece used in torque, transducersensitivity testing; and

FIG. 9 is a graphical illustration, as in FIG. 7, of the sensitivity vs.residual stress loading relationship for a transducer of the presentinvention wherein the bands thereof were endowed with residual stressinduced magnetic anisotropy by a controlled knurling technique.

FIGS. 10, 11 and 12 are graphica1 representations of the relationshipbetween applied torque and output signal for magnetoelastic torquetransducers made in accordance with the present invention.

FIG. 13 is a schematic representation of an exemplary device, e.g.,internal combustion engine, power transmission means or fluid turbinemeans, incorporating the magnetoelastic torque transducer of the presentinvention.

FIG. 14 is a schematic representation of a weighing system incorporatingthe magnetoelastic torque transducer of the present invention.

FIG. 15 is a schematic representation of a machine tool incorporatingthe magnetoelastic torque transducer of the present invention.

FIG. 16 is a schematic representation of a robotic device incorporatingthe magnetoelastic torque transducer of the present invention.

FIG. 17 is a schematic representation of a vehicular steering systemincorporating the magnetoelastic torque transducer of the presentinvention.

FIG. 18 is a schematic representation of a force measuring systemincorporating the magnetoelastic torque transducer of the presentinvention.

Best Mode For Carrying Out The Invention

In accordance with the present invention there is provided amagnetoelastic torque transducer comprising (1) a torque carrying memberat least the surface of which, in at least one complete circumferentialregion of suitable axial extent, is appropriately ferromagnetic andmagnetostrictive; (2) two axially distinct circumferential bands withinthis region or one such band in each of two such regions that areendowed with respectively symmetrical, helically directed residualstress induced magnetic anisotropy such that, in the absence of torque,the magnetization tends to be oriented along a left-hand (LH) helix inone band and along an axially symmetrical right-hand (RH) helix in theother band; and (3) a magnetic discriminator device for detecting,without contacting the torqued member, differences in the response ofthe two bands to equal, axial magnetizing forces.

These features of the magnetoelastic torque transducer of the presentinvention will be better understood by reference to FIG. 1 in which acylindrical shaft 2 formed of ferromagnetic and magnetostrictivematerial or, at least having a ferromagnetic and magnetostrictive region4, is illustrated having a pair of axially spaced-apart circumferentialor annular bands 6,8 endowed with respectively symmetrical, helicallydirected magnetic stress anisotropy in the angular directions ± θ of therespective magnetic easy axes 10,12. A magnetic discriminator 14 isspaced from shaft 2 by a small radial space. In the absence of appliedtorque the magnetization within the bands 6,8 will respond symmetricallyto the application of equal axial magnetizing forces. Longitudinal oraxial components of the magnetization within these two bands remainidentical, since cos θ=cos-(θ) for all values of θ, and the magneticdiscriminator will therefore, detect no difference or zero. With theapplication of torque to shaft 2, the stress anisotropy arisingtherefrom combines asymmetrically with the quiescent anisotropiesintentionally instilled in the bands and there is then a differingresponse of the two bands to equal axial magnetizing force. Since thestress anisotropy is a function of the direction and magnitude of thetorque, the differential response of the two bands will be a monotonicfunction of the torque. The resulting differences in magnetic anisotropyin each of the bands is evidenced by the axial permeability of one bandincreasing and that of the other band decreasing. The difference inaxial permeabilities of the two bands is used to sense the torque. Aproperly designed magnetic discriminator will detect detailed featuresof the differential response and provide an output signal that is ananalog of the torque.

In accordance with the present invention, the torque carrying member isprovided with two axially spaced-apart, distinct circumferential orannular bands in the ferromagnetic region of the member. There are noparticular geometric, space, location or circumferential limitations onthese bands, save only that they should be located on the same diametermember and close enough to one another to experience the same torque.The bands are intentionally endowed with respective symmetrical,helically directed, magnetic anisotropy caused by residual stress.Residual stress may be induced in a member in many different ways, asdiscussed more fully hereinbelow. However, all techniques have in commonthat they apply stress to the member beyond the elastic limit of atleast its surface region such that, when the applied stress is released,in the absence of external forces, the member is unable to elasticallyreturn to an unstressed condition. Rather, residual stresses remainwhich, as is well known, give rise to magnetic anisotropy. Dependingupon the technique utilized for applying stress, the angular directionof the tangential principal residual stress with the member's axis willvary between values greater than zero and less than 90° . Preferably,the angular direction of the residual stress and that of the resultingmagnetic easy axes, is from 10° -80° and, most desirably, from 20° -60°.

As used herein, "residual stresses" are those stresses that exist in abody in the absence of external forces. Their distribution is such thatthe net forces and moments acting on various regions within the body sumto zero on the whole body. Thus, if any one region exerts net forcesand/or torques on the remainder of the body then other regions mustexert compensating forces and/or torques. The consequence of thisrequirement for residual stresses to establish self-compensatingdistributions is that the existence of any region experiencing a tensilestress implies the existence of a communicating region experiencingcompressive stress. Residual stress is often classified by the size ofthe regions and the distances separating compensating regions as shortrange (SR) and long range (LR). SR stresses exist in regions from a fewatoms up to a size comparable to microstructural features such as asingle grain. LR stresses exist over dimensions from more than one grainto macroscopic features of the whole body.

The requirement of the present invention for a relatively coherentmagnetic anisotropy over a major proportion of a circumferential band ofmacroscopic axial extent can be met with deliberately instilled,appropriately distributed LR stresses. In any one band the principalcomponents of the residual stress at and near the surface are relativelyuniform throughout the band and characterizable by a helicaldirectionality. The stresses necessary to compensate for this surfacestress should desirably lie radially inwards from the surface so as tobe undetectable by surface magnetic sensing. This same distribution, butwith an opposite handed, equi-angled helicity is instilled in thecooperative band.

It will be appreciated that inasmuch as the sensing of torque isprimarily accomplished by sensing the change in permeability at thesurface of the torqued member, it is at least at the surface of eachband that there must be magnetic anisotropy created by residual stress.Hence, the limitation that the applied stress must be at leastsufficient to exceed the elastic limit of the member at its surface. Itwill, of course, be appreciated that the application of an appliedstress exceeding the minimum will, depending upon the magnitude of theapplied stress, result in residual stress within the body of the memberas well. For use herein, the term "surface" of the member means at thesurface and within 0.010 inch thereof.

Any method of applying stress to a member to exceed the elastic limitthereof at the surface of the bands may be employed which producesuneven plastic deformation over the relevant cross-section of themember. Thus, the residual stress inducing method may be mechanical,thermal, or any other which is suitable. It is particularly desirablethat the residual stress-inducing applied stress exceed the maximumexpected applied stress when the member is torqued in use. This is toinsure that torquing during use does not alter the residual stresspattern and, thus, the magnetic anisotropy within the bands. Theresidual stress induced in the respective bands should be substantiallyequal and symmetrical in order that axial permeability sensing, whenequal axial magnetizing forces are applied to the member, will produce a"no difference" output in the untorqued condition and equal but oppositeoutput as a result of the application of equal clockwise (CW) andcounter-clockwise (CCW) torques.

The method chosen to apply stress to a member beyond the elastic limitthereof in order to create residual stress is largely a function of themember's size, shape, material and intended application. The method mayinduce continuous and substantially equal residual stresses over theentire surface of the band, i.e., around the entire band circumferenceand along its entire axial length. Alternatively, the method may inducea residual stress pattern within each band which includes both stressedand unstressed areas. Such a pattern, however, is subject to theimportant limitation that each band must have at least one continuouscircumferential region which is free of unstressed area s over at least50% of its circumferential length, desirably over at least 80% of itscircumferential length. In a particularly preferred configuration, eachband would have at least one continuous circumferential region which isfree of unstressed areas over its entire circumferential length. As ageneral matter, it is particularly desirable to maximize the amount ofshaft surface which is intentionally stressed in order to endow as muchof the surface as is possible with relatively large magnitude controlledmagnetic anisotropy. This leaves as little of the shaft surface aspossible subject only to the random anisotropies created during shaftmanufacture, due to internal stresses and resulting from crystalorientation. It should be appreciated that the problems associated withrandom anisotropy inherent in using the shaft itself as an operativeelement, i.e., the sensing region, of the magnetic circuit of the torquesensor are overcome, in accordance with the present invention, byreplacing and/or overwhelming the random anisotropy with relativelylarge magnitude intentionally created residual stress inducedanisotropy. For obvious reasons, the greater the intentionally inducedanisotropy, the less significant is any residual random anisotropy.

As used hereinbefore and hereinafter, the term "circumferential region"means the locus of points defining the intersection of (1) a planepassing perpendicular to the member's axis and (2) the surface of themember, as hereinbefore defined. Where the member is a cylindricalshaft, the circumferential region is a circle defining the intersectionof the cylindrical surface with a plane perpendicular to the shaft axis,and such a circle has a circumference or circumferential length. Statedotherwise, if each element of the member's surface comprising thecircumferential region were examined, it would be seen that each suchelement was either stressed or unstressed. In order to form acommercially functional torque sensor having broad applicability,particularly in small diameter shaft applications, which exhibitsacceptable and commercially reproducible sensitivity, linearity andoutput signal strength, it has been found that at least 50% of theseelements must have been stressed beyond their elastic limit and,therefore, must remain residually stressed after the applied stress isremoved.

The range of methods by which torque carrying members can be endowedwith the desired bands containing residual stress instilled helicallydirected magnetic easy axes, i.e., directions in which magnetization iseasiest, is virtually endless. From the point of view of transducerperformance the most important consideration is the adequacy of theresulting anisotropy, i.e., the band anisotropy created must be at leastof comparable magnitude to the stress anisotropy contributed by theapplied torque. From the point of view of compatibility with the devicein which the transducer is installed, the compelling consideration isconsequential effects on the member's prime function. Other importantconsiderations in selecting a method are practicality and economics.Examples of suitable methods for imprinting residual stress inducedmagnetically directional characteristics on, i.e., at the surface of, atorque carrying member include, but are not limited to, torsionaloverstrain; knurling; grinding; mechanical scribing; directed or maskedshot peening or sand blasting; roll crushing; appropriate chemicalmeans; selective heat treatments, e.g., induction, torch, thermal printhead, laser scribing.

Of the foregoing, the creation of areas of residual stress by torsionaloverstrain has been found to be a simple, economical and effectivemethod for small diameter shafts. It is particularly desirable becauseit neither distorts nor interrupts the surface of the shaft and is,therefore, compatible with virtually any application. However, themanner of applying torsional overstrain, e.g., by twisting both sides ofa centrally restrained region, makes it impractical for and inapplicableto large diameter shafts formed of high elastic limit materials.Knurling is a desirable manner of inducing residual stress in a shaft ofvirtually any diameter. With knurling, the exact location of the bands,their axial extent, separation and location can be closely controlled.In addition, knurling allows relatively simple control of the helixangles of the easy axes. Very importantly, knurling permitspredetermination of the salient features of the knurl itself, such aspitch, depth and cross-sectional shape and, thereby, allows control ofthe residual stress induced. It should be appreciated that, inaccordance with the present invention, enough of the surface of eachband must be stressed that there exists within each band at least onecontinuous circumferential region which is free of unstressed areas overat least 50% of its circumferential length. Not all knurling is thisextensive and care must be taken to select a knurl and a method forapplying the knurl which achieves this objective. Inasmuch as knurlingdisrupts the surface of the shaft in order to form the knurl thereon, aknurled band is endowed with shape anisotropy as well as residual stressanisotropy. If it is desired, for example, for compatibility of theknurled shaft with an intended application, the gross shape features ofthe knurl may be ground off the shaft to leave only magnetic anisotropycaused by residual stress. 0f course, knurling is not without itssubstantial shortcomings and, as will be seen hereinafter, is limited inits application to alloys having particular properties. Other forms ofcold working, with or without surface deformation, such as grinding,likewise create residual stress and associated magnetic anisotropy andare advantageous forming processes in the manufacture of torque sensorsin accordance with the present invention. In addition, moresophisticated methods, such as electron beam and laser scribing as wellas selective heat treatment can provide the desired anisotropy with lessmutilation of the shaft surface than most mechanical cold workingmethods. Moreover, these methods offer the opportunity of very closecontrol of the induced residual stresses by adjustment of the powerdensity and intensity of the beam and/or the thermal gradients.

Whatever method may be selected for creating residual stress within thebands, it should be appreciated that the relationship between thepercent of stressed areas along the circumferential length of acircumferential region within each band ("% stressed areas") andsensitivity (in millivolts/N-M) is one wherein the sensitivity increaseswith increasing "% stressed areas". A plot of these parameters yields acurve which has its greatest slope at the lower values of "% stressedareas" and which has a decreasing slope at the higher values of "%stressed areas", up to 100%, at which point the sensitivity is greatestand the slope is close to zero. The precise shape of the curve, itsslope for any particular value of "% stressed areas", its initial rateof ascent and the point at which the rate of ascent decreases and thecurve levels off are all functions of the material of the bands and themanner in which the stress is applied. A typical curve is shown in FIG.7. At "A", there is no residual stress along the circumferential lengthof the circumferential region. At "C", 100% of the circumferentiallength of the circumferential region is subjected to residual stress."B" represents the approximate point on the curve at which sensitivitybegins to level off, i.e., becomes less responsive to "% stressedareas," a point which is both material and method dependent.

Ideally, torque sensor operation at 100% residual stress, i.e., at "C"on the curve, is best because the rate of change of sensitivity isminimized and the 100% stressed condition is generally easiest to attainwith most methods. As a practical matter, it is difficult to control theresidual stress inducing method to achieve a value for desired "%stressed area" which is less than 100%. However, practical productionproblems aside, acceptable torque sensors can be made which operate atsensitivity levels corresponding to less than 100% residual stress alongthe length of a circumferential region of the bands.

Torque sensors cannot economically and reproducibly be made to operatein the ascending portion AB along the curve in FIG. 7 since, in thatportion, the sensitivity is extremely responsive to "% stressed areas".This means that even small changes in "% stressed areas" causesrelatively large changes in sensitivity. From a practical, commercialstandpoint, mass produced torque sensors must have a known andreproducible sensitivity. It would be unrealistic to have toindividually calibrate each one. However, even normal productioninconsistencies will cause small "% stressed areas" changes which willresult, in the AB region of the curve, in large sensitivity differencesamong sensors. Therefore, commercially useful torque sensors have tooperate along a flatter portion of the curve, where the slope is closerto zero. Operating in the BC portion of the curve appears to be anacceptable compromise. It is preferred, for most materials and residualstress inducing methods, that the point represented by "B" exceed atleast 50%, preferably at least 80%, stressed areas along thecircumferential length of a circumferential region. This is inrecognition of the fact that the minimum acceptable residual stressloading of a circumferential region is both material and processdependent and that it is generally most desirable to be as close to 100%stress loading as is practical.

To demonstrate the applicability of the foregoing in fabricating anoperable torque sensor, with reference to FIG. 8, a 0.25 inch ODcylindrical shaft 100 was formed with two shoulders 102 of equal axiallength spaced apart by a reduced diameter shaft portion 104 of 0.215inch OD. The shaft was formed of a nickel maraging steel commerciallyavailable as Unimar 300K from Universal-Cyclops Specialty SteelDivision, Cyclops Corporation of Pittsburgh, Pa. and was pre-annealed at813° C. in hydrogen to relieve internal stresses. Each shoulder 102 wascarefully knurled using a pair of identical 3/4 inch OD, 3/8 inch longknurling rollers having 48 teeth around their circumference. Theshoulders were brought into contact with the knurling rollers in acontrolled manner to form symmetrical knurls on each shoulder at anglesof ± 30° to the shaft axis. Careful control of the infeed of the toolrelative to the shoulders allowed the axial width and depth of eachknurl trough to be controlled. The "% stressed areas" along thecircumferential length of a circumferential region of each knurledshoulder was determined by assuming that the knurl trough was the onlystressed area on the shoulder and that the shoulder surface betweentroughs was unstressed by the knurling operation; by measuring thetrough width and chordal knurl pitch and converting the chordal pitch tocircumferential pitch; and by calculating the trough width tocircumferential pitch ratio, which ratio when multiplied by 100represented the desired "% stressed areas" value. The shaft prepared inthis manner was affixed to a lever arm which permitted 10-one poundweights to be suspended from cables at each end of the arm. The leverarm was so dimensioned that addition or removal of a single one poundweight from either side represented a torque change on the shaft of 0.5N-M. By appropriate shifting of the weights, the torque on the shaftcould be altered in both magnitude and direction.

FIG. 9 graphically illustrates the relationship between "% stressedareas" and sensitivity for a shaft prepared as described hereinabove. Itcan be seen that the curve ascends rapidly up to about 60% stressloading and then appears to level off rather rapidly thereafter. This isbecause there is believed to be a greater correlation at lower "%stressed area" values between the trough width to circumferential pitchratio and the actual percentage of stressed areas along thecircumferential length of a circumferential region of the shaft. As thewidth and depth of the knurling trough increases it becomes apparentthat the shoulder surface between troughs, at least in the vicinity ofthe trough edges, becomes slightly deformed and, more than likely,residually stressed. Therefore, the point on the curve at which 100%stress loading in a circumferential region is actually achieved issomewhat less than the calculated 100% value, accounting for the rapidflattening of the curve at the higher "% stressed areas" portionsthereof. This suggests that, with many processes, such as knurling, the100% stress loading point can be achieved with less than 100%topographic disruption. It will be appreciated in this connection, thateach method of inducing residual stress in a shaft will produce its owndistinctive curve of "% stressed areas" vs. sensitivity, although it isbelieved that each curve will have the same general characteristics asappear in FIGS. 7 and 9.

In accordance with the foregoing, it can be seen that in the absence ofapplied torque, the application to the bands of equal axial magnetizingforces causes the bands to respond symmetrically and the sensing meansassociated with the bands detect no difference in response. When torqueis applied, the principal stresses associated with the applied torquecombine with the residual stresses in the bands in such a manner thatthe resultant stresses in the two bands are different from each other.As a result, the magnetic permeabilities are different and the emfinduced in the sensing means associated with each band reflect thatdifference. The magnitude of the difference is proportional to themagnitude of the applied torque. Thus, the instant system senses adifferential magnetoelastic response to the principal stressesassociated with the applied torque between two circumferential bands.The significance of this is that sensing in this manner amounts tosensing the response averaged over the entire circumference of the band.In this manner, sensitivity to surface inhomogenity, position androtational velocity are avoided.

This sensing of magnetic permeability changes due to applied torque canbe accomplished in many ways, as is disclosed in the prior art. See, forexample, the aforementioned article of Harada et al and U.S. Pat. No.4,506,554. Functionally, the magnetic discriminator is merely a probefor assessing any differential magnetoelastic response to applied torquebetween the two bands. In general, it functions by imposing equalcyclically time varying magnetizing forces on both bands and sensing anydifferences in their resulting magnetization. The magnetizing forces maycome from electrical currents, permanent magnets, or both. Resultingmagnetization may be sensed through its divergence, either by theresulting flux or its time rate of change. The transducer function iscompleted by the electrical circuitry which delivers an electricalsignal that is an analog of the torque.

One method of supplying the magnetization forces and for measuring theresulting difference signal from the sensing coil is shown in FIGS. 2and 3. Referring to FIG. 2, it can be seen that the bands 6,8 aresurrounded by bobbins 16,18 which are concentric with shaft 2. Mountedon bobbins 16,18 are a pair of coils 20,22 and 24,26 of which 22 and 26are excitation or magnetizing coils connected in series and driven byalternating current and 20 and 24 are oppositely connected sensing coilsfor sensing the difference between the fluxes of the two bands. Aferrite material core 28 is optionally provided as a generally E-shapedsolid of revolution. Circumferential gaps 30 between the shaft and theE-shape core are desirably maintained as small and uniform as ispractical to maintain the shaft centered within the core. FIG. 3 showsthat excitation or drive coils 22,26 are supplied in series from ACsource 32 and the emf induced in the oppositely connected sensing coils20,24 is phase sensitively rectified in the rectifier 34 and isdisplayed on voltage display instrument 36. Black dots 38 indicate thepolarity of the coils.

Inasmuch as the stresses in the bands are symmetrical and equal when notorque is applied to shaft 2, under these conditions the output signalfrom the circuitry shown in FIG. 3 will be zero, regardless of theapplied a.c. driving input. This is because the bands have equalmagnetic permeability. Thus the voltages induced in the sensing coilsare equal in magnitude and opposite in polarity and cancel each other.However, when a torque is applied to shaft 2, the respective bands willbe subjected to tensile and compressive stresses, with a resultingincrease of permeability and of the flux passing through one of thebands, and a resulting decrease of permeability and of the flux passingthrough the other of the bands. Thus, the voltage induced in one of thesensing coils will exceed the voltage induced in the other sensing coiland an output signal representing the difference between the inducedvoltages and proportional to the applied torque will be obtained. Thesignal is converted to a direct current voltage in the rectifier 34 andthe polarity of the rectifier output will depend upon the direction,i.e., CW or CCW, of the applied torque. Generally, it has been foundthat in order to obtain linear, strong output signals, the a.c. drivingcurrent should advantageously be maintained in the range 10 to 400milliamperes at excitation frequencies of 1 to 100 kHz.

FIG. 4 illustrates another type of magnetic discriminator for sensingthe permeability change of the bands upon application of a torque to theshaft. Magnetic heads 42,44 comprising a ferromagnetic core and a coilwound thereupon are provided in axial locations along shaft 40 whichcoincide with bands 46,48 and are magnetically coupled to the bands. Themagnetic heads 42,44 are excited by high frequency power source 50through diodes 52,54. With no torque applied to shaft 40, the magneticpermeability of the bands are equal. Therefore, the inductance levels ofboth magnetic heads are equal and opposite in polarity, and the netdirect current output, V_(out), is zero. When torque is applied to shaft40, as shown by arrows 60, the magnetic permeability of one bandincreases while the permeability of the other decreases.Correspondingly, the inductance of one magnetic head increases while theinductance of the other decreases, with a resultant difference inexcitation current between the heads. This difference in excitationcurrent, passed via output resistors 56 and smoothing capacitor 58,produces a direct current output signal which has polarity and magnitudeindicative of the magnitude and direction of the applied torque.

In accordance with one unique aspect of the present invention, ashereinbefore described, a shaft of suitable material is endowed in eachof two proximate bands with symmetrical, left and right handed helicalmagnetic easy axes. At least in the region of the bands, and morecommonly over its entire length the shaft is formed, at least at itssurface, of a material which is ferromagnetic and magnetostrictive. Thematerial must be ferromagnetic to assure the existence of magneticdomains and must be magnetostrictive in order that the orientation ofthe magnetization may be altered by the stresses associated with anapplied torque. Many materials are both ferromagnetic andmagnetostrictive. However, only those are desirable which also exhibitother desirable magnetic properties such as high permeability, lowcoercive force and low inherent magnetic anisotropy. In addition,desirable materials have high resistivity in order to minimize thepresence of induced eddy currents as a result of the application of highfrequency magnetic fields. Most importantly, favored materials mustretain these favorable magnetic properties following the cold workingand heat treating necessary to form them into suitable shafts havingappropriately high strength and hardness for their intended use.

It is true that many high strength steel alloys are ferromagnetic andmagnetostrictive. However, to varying degrees, the vast majority ofthese alloys experience a degradation in their magnetic properties as aresult of the heat treating necessary to achieve suitable hardness andstrength for the desired application. The most significant degradationis noted in those alloys hardened by carbon or carbides for which theconventional inverse relationship between mechanical hardness andmagnetic softness appears to have a sound basis. However, theperformance of even low carbon alloys such as AISI 1018 is found tosignificantly degrade with heat treating. The same is true formartensitic stainless steels, e.g., AISI 410, and highly alloyed steels,e.g., a 49Fe-49Co-2V alloy. It has been determined, in accordance withanother unique aspect of the present invention, that the nickel maragingsteels possess the unusual combination of superior mechanical propertiesand outstanding and thermally stable magnetic properties which give thema special suitability and make them particularly advantageous for use inall magnetoelastic torque transducers in which a magnetic field isapplied to ferromagnetic, magnetostrictive means and the change inpermeability caused by torque applied thereto is sensed to obtain anindication of the magnitude of the applied torque. This is the casewhether the ferromagnetic, magnetostrictive means is affixed to,associated with or forms a part of the surface of the torqued member andwhether or not the ferromagnetic, magnetostrictive means is endowed withbands of intentionally instilled magnetic anisotropy and irrespective ofthe number of bands which may be used.

The nickel maraging steels are, typically, extra-low-carbon, highnickel, iron-base alloys demonstrating an extraordinary combination ofstructural strength and fracture toughness in a material which isreadily weldable and easy to heat-treat. They belong to a loosely knitfamily of iron-base alloys that attain their extraordinary strengthcharacteristics upon annealing and during cooling, by transforming to aniron-nickel martensitic microstructure, and upon aging in the annealedor martensitic condition. Thus, the alloys are termed "maraging" becauseof the two major reactions involved in their strengthening--martensitizing and aging. However, these steels are unique due to theirhigh nickel and extremely low carbon content, which permits formation ofan outstandingly tough martensite that can be strengthened rapidly toextraordinarily high levels. Yield strengths up to and well beyond 300ksi are available in these steels in the aged condition.

Typical nickel maraging steels are alloys comprising 12-25% Ni, 7-13%Co, 2.75-5.2% Mo, 0.15-2.0% Ti, 0.05-0.3% A1, up to 0.03% C, balance Feand incidential amounts of other elements, such as Mn, Si, S, P, Cb. Themost popular and practically significant maraging steels, at least atpresent, are the 18% Ni steels which can be aged to develop yieldstrengths of about 200 ksi, 250 ksi and 300 ksi. These particularalloys, referred to as 18Ni200, 18Ni250 and 18Ni300 grade maragingsteels have typical compositions in the ranges 1-19% Ni, 7-9.5% Co,3.0-5.2% Mo, 0.1-0.8% Ti, 0.05-0.15% A1, up to 0.03% C, balance Fe andincidential amounts of other elements. Typically, the 18% nickelmaraging steels are heat treated by annealing at temperatures of 1500°F. and above for a sufficient time, e.g., one hour per inch ofthickness, to dissolve precipitates, relieve internal stresses andassure complete transformation to austenite. Following air cooling, the18% Ni steels are conventionally aged at 750-1100° F., desirably900-950° F., for 3 to 10 hours, depending upon thickness, usually 3-6hours. However, it has been found that satisfactory strengthcharacteristics and superior magnetic characteristics can be attained inalloys aged for as little as 10 minutes.

Other well know n nickel maraging steels are cobalt-free 18% Ni maragingsteels as well as cobalt-containing 25% Ni, 20% Ni and 12% Ni maragingsteels. The 18% Ni-cobalt containing maraging steels are commerciallyavailable from a number of sources. Thus, such steels are obtainableunder the trademarks VascoMax C-200, VascoMax C-250, VascoMax C-300 andVascoMax C-350 from Teledyne Vasco of Latrobe, Pa.; under the trademarksMarvac 250 and Marvac 300 from Latrobe Steel Company of Latrobe, Pa.;under the trademark Unimar 300K from Universal-Cyclops Specialty SteelDivision, Cyclops Corporation of Pittsburgh, Pa.; and, under thetrademark Almar 18-300 from Superior Tube of Norristown, Pa. The 18%Ni-cobalt free maraging steels are commercially available under thetrademarks VascoMax T-200, VascoMax T-250 and VascoMax T-300 fromTeledyne Vasco of Latrobe, Pa. Other high nickel steels which form aniron-nickel martensite phase exhibit mechanical and magnetic propertieswhich are similar to those of the more conventional maraging steels andwhich are also substantially stable to temperature variations. Mostnotable among these is a nominally 9% Ni-4% Co alloy available fromTeledyne Vasco having a typical composition, in percent by weight, of9.84 Ni, 3.62 Co, 0.15 C, balance Fe. In addition, maraging steels ofvarious other high nickel-cobalt compositions, e.g., 15% Ni-15% Co, arecontinuously being tested in efforts to optimize one or another or somecombination of properties. Therefore, as used herein, the term "Nimaraging steel" refers to alloys of iron and nickel which contain from9-25% nickel and which derive their strength characteristics fromironnickel martensite formation, as hereinbefore described.

In addition to their outstanding physical and strength characteristics,the nickel maraging steels have excellent magnetic properties which makethem outstanding for use as the magnetic material in non-contact torquetransducers. Thus, they have high and substantially isotropicmagnetostriction, in the range of 25 ppm 15 ppm, and do not exhibit aVillari reversal; high electrical resistivity; low inherent magneticanisotropies due to crystalline structure; high magnetic permeability;low coercive force, in the range 5-25 oersted; and, stability ofmagnetic properties with alloy chemistry. However, most important isthat their magnetic properties are only modestly, yet favorably,affected by strengthening treatments--indeed, their magnetic propertiesimprove with cold work and aging heat treatment. This characteristicdistinguishes the nickel maraging steels from all other high strengthalloys. Heretofore, it had been the conventional wisdom that the heattreatments needed to improve the mechanical and strength properties ofsteels were detrimental to their magnetic properties. For example,quench hardened steel alloys typically exhibit very low magneticpermeabilities and high coercive forces, a combination of unfortunatemagnetic properties which materially decrease the sensitivity of suchalloys to small magnetic fields and diminish or negate their usefulnessin torque transducers such as are contemplated herein. This isdemonstrably not the case with the nickel maraging steels. In accordancewith the present invention it has been determined that nickel maragingsteels get magnetically softer following cold work and the aging heattreatments to which they are conventionally subjected in order todevelop their extraordinary high strength characteristics. For example,the coercive force of an 18% Ni maraging steel in fact decreases whenaged at 900° F. for up to 10 hours. As a result the maraging steels canbe advantageously used in their aged condition, i.e., in a conditionwhere they exhibit maximum strength characteristics and substantiallythe same or improved magnetic characteristics.

Thus, the use of maraging steels as the magnetic material in amagnetoelastic torque sensor, particularly as the shaft material in adevice whose torque is to be sensed, obviates virtually all of theobjections heretofore made to using the device shaft as the magneticmember. The mechanical and strength properties of maraging steelssatisfy the mechanical properties requirements for most all shaftapplications while, at the same time, providing outstanding magneticproperties for its role in the torque sensor. Aging of the maragingsteels provides the high strength and high hardness needed for themechanical application without loss of magnetic permeability or increasein coercive force. Moreover, the conventional manner of heat treatingmaraging steel, including the initial solution anneal at temperatures inexcess of 1500° F., relieves internal stresses due to mechanical workingand most stresses due to inhomogeneities and crystal orientation, thusminimizing the amount of random magnetic anisotropy in a maraging steelshaft. When such heat treatment is combined with the creation, accordingto the present invention, of a pair of adjacent bands endowed withintentionally instilled magnetic stress anisotropy of a relatively largemagnitude, e.g., by stressing the shaft beyond its elastic limits withapplied stresses of a magnitude greater than the largest torque stressesanticipated during normal usage of the shaft, the contribution to totalmagnetic anisotropy of any random anisotropy in the shaft is indeednegligible.

It will be appreciated that the advantage of the nickel maraging steelsin magnetoelastic torque transducers can be realized by forming theshaft of the desired nickel maraging steel, by forming a region of theshaft of the desired nickel maraging steel and locating the annularbands within this region, or by surfacing with a nickel maraging steel ashaft formed of an alloy having mechanical properties suitable for theintended function of the shaft, i.e., applying over at least onecomplete circumferential region of suitable axial extent of the shaft asurfacing alloy of the desired nickel maraging steel and locating theannular bands within this region. Inasmuch as magnetic permeabilitysensing in accordance with the present invention is fundamentally asurface phenomena, the surfacing process need apply a circumferentiallayer of thickness not exceeding about 0.015 inches. The surfacingprocess selected may advantageously be selected from among the manyknown additive processes, e.g., electroplating, metal spraying,sputtering, vacuum deposition, ion implanatation, and the like.

In order to demonstrate the outstanding qualities of the maraging steelsas the magnetic material in torque transducers of the present inventionand to compare the performance of maraging steels with other highstrength steels, a torque transducer was assembled using a 12.7 mmdiameter cylindrical shaft having formed thereon a pair of axiallyspaced-apart bands endowed with helically symmetrical LH and RH magneticeasy axes. The bands each had an axial length of 12.7 mm and wereseparated by a 12.7 mm shaft segment. They were formed by knurling usinga 3/4-inch OD knurling tool having 48 teeth around the circumference,each tooth oriented at 30° to the shaft axis. The characteristics ofthis arrangement were sensed by positioning bobbins concentric with theshaft and axially aligned with the bands, each bobbin having amagnetizing and sensing coil mounted thereon. The magnetizing coils wereconnected in series and driven by an alternating current source having a10 KHz frequency output and a 200 mA peak driving current. The emfinduced in each of the sensing coils was separately rectified with therectified outputs oppositely connected to produce a difference signalwhich was displayed on a voltage display instrument. Four shafts wereemployed, identical in all respects except they were each formed ofdifferent materials. The composition of each shaft is set forth inpercent by weight hereinbelow:

T-250: 18.5 Ni; 3.0 Mo; 1.4 Ti; 0.10 A1; less than 0.03 C; no cobalt;balance Fe

SAE 9310: .08-0.13 C; 0.45-0.65 Mn; 3-3.5 Ni; 1-1.4 Cr; 0.08-0.15 Mo;balance Fe

416 SS: 11.5-13.5 Cr; 0.5 max Ni; 0.15 max C; 1.0 max Mn; 1.0 max Si;balance Fe

AISI 1018: 0.15-0.20 C; 0.6-0.9 Mn; 0.04 max P; 0.05 max S; balance Fe

In a first series of runs, the T-250 nickel maraging steel shaft wasused in the solution annealed, unaged condition as received fromTeledyne Vasco. Likewise, the other shafts were also used in theiraspurchased condition without further heat treatment. A known torqueloading was applied to each shaft under test and the output voltagesignal was recorded. The applied torque was increased from zero up to100 newton-meters (N-M). FIG. 5 is a graph of applied torque versusoutput d.c. voltage for each shaft. It is apparent that the sensitivityof the T-250 shaft in terms of magnitude of output signal for a giventorque loading was significantly greater than for the other shaftmaterials tested. In addition, the linearity of the output signal forthe T-250 shaft was extremely good over the entire torque range. Theother shaft materials appeared to be about equally insensitive, comparedto the T-250 shaft, to applied torque. None produced as linear a signalas the T-250 shaft, although each produced a reasonably linear signalover most of the torque range.

For the second series of runs, the T-250 nickel maraging steel shaft wasaged at about 900° F. for 30 minutes to improve the strength andhardness of the shaft. For consistency of testing, the other shafts wereheat treated in the same manner, after which each shaft was subjected toan applied torque from zero to 100 N-M and the output d.c. voltagerecorded. FIG. 6 is a graph of applied torque versus output d.c. voltagefor each shaft after heat treatment. It can be seen that once again thesensitivity of the T-250 shaft far exceeded the sensitivity of the othershafts and once again the T-250 output signal was linear over the entiretorque range. By comparison with FIG. 5 for the T-250 shaft in theunaged condition it is apparent that aging measurably improved thesensitivity of the shaft, indicating an enhancement of the magneticproperties of the maraging steel with aging. By contrast, thesensitivity of the SAE 9310 shaft did not appear to improve with thisheat treatment. Moreover, the linearity of the output signal clearlydegraded, particularly at higher applied torques. The sensitivity of theAISI 1018 shaft significantly improved at low applied torques but theimprovement began to abate at about 40 N-M and degraded thereafter. Thelinearity of the output signal for the aged AISI 1018 shaft was verypoor. For the 416 SS shaft, the sensitivity at low applied torquesimproved with heat treatment but significantly worsened at higherapplied torques. The linearity of the 416 SS output signal became worsewith heat treatment. It is noteworthy that notwithstanding the mixedresponse of the output signal to applied torque, heat treatmentadversely affected the mechanical and strength properties of the SAE9310, 416SS and AISI 1018 shafts. For example, following heat treatment,an applied torque of only about 50 N-M exceeded the elastic limit of theAISI 1018 shaft and the shaft permanently twisted.

Moreover, the results reported in FIG. 6, although significant foreffecting a comparison with the T-250 alloy, are somewhat deceptive interms of evaluating the actual usefulness of these other alloys in theshafts of magnetoelastic torque transducers. This is because the heattreatment to which these alloy s were subjected was aging at 900° F. for30 minutes, the same heat treatment used for the T-250 alloy. However,such a heat treatment is not an effective heat treatment for improvingthe mechanical and strength properties of these steel alloys. Typically,for example, quench hardening of 416SS requires heating to above 950° C.and case hardening of SAE 9310 requires heating to above 900° C. Atthese high temperatures, a large proportion of the residual stresscreated within the bands by the knurling process and the resultingresidual stress-created magnetic anisotropy is removed, leaving amechanically hard but magnetically inferior alloy which is essentiallyuseless as the magnetostrictive, ferromagnetic shaft element in themagnetoelastic torque transducers of the present invention. In addition,it has been noted that certain thermal hardening treatments, such ascase hardening SAE 9310, tend to warp the shaft which renders the shaftuseless, without further processing, for its intended as well as anytransducer purpose. Unfortunately, the necessary further processing tostraighten the warped shaft, such as machining, will alter theintentionally instilled magnetic anisotropies in the bands, thuseffectively undoing the careful magnetic preconditioning of the shaft.As will become clearer from the discussion which follows, the betterprocedure for preparing a magnetoelastic torque transducer in accordancewith the present invention from a non-Ni maraging steel alloy such asSAE 9310 is to first thermally harden the alloy, as by case hardening,and then machine the resulting shaft as needed to straighten it.Thereafter, grinding or other processing of the hardened alloy can beaccomplished to provide bands endowed with residual stress createdmagnetic anisotropy. One advantage of the use of grinding is that itremoves any surface anisotropies instilled by the straightening processand substitutes the desired residual stress-created anisotropies, allwithout macroscopic topographic alteration of the shaft surface.

FIGS. 5 and 6 graphically illustrate the signal response to appliedtorque using a relatively low, 10 kHz, a.c. excitation frequency. It hasbeen found that the output signal is directly proportional to andincreases approximately linearly with a.c. frequency. Tests show that at20 kHz, for example, a doubling of the output d.c. voltage signal isobtained. Depending upon the circuitry employed, a.c. frequencies in therange 1-100 kHz can advantageously be used to drive torque transducersof the present invention. Preferably, frequencies of 10-30 kHz, justabove the human audible range, are used in order to avoid whistling.Most desirably, the frequency is adjusted to about 20 kHz. Like itsresponse to frequency, the output d.c. signal also appears to bedirectly proportional to, more specifically to vary sigmoidally with,the drive current which, depending upon the frequency, can usefully bein the range 10-400 mA (peak). Generally, sufficient current is used toobtain a good signal at the chosen frequency and, desirably, to adjustthe signal hysteresis to zero over the entire applied torque range.

It is interesting to note that the sensitivity of a nickel maragingsteel shaft is markedly better than the sensitivities reported byworkers employing nonmagnetic shafts and adhesively affixing amorphousribbons thereto. From FIG. 6, it can be seen that according to thepresent invention an aged T-250 nickel maraging steel shaft transducer,having a shaft diameter of 12.7 mm, produces an output d.c. signal of0.9 volts at an applied torque of 60 N-M using an a.c. frequency of 10kHz and an exciting current of 200 mA and employing exciting coilshaving 100 turns each and sensing coils having 500 turns each, asensitivity of 0.015 V/N-M. By comparison, Sasada et al, in the paper"Noncontact Torque Sensor", presented at the 11th Annual IEEE IndustrialElectronics Society Conference (Nov. 18-22, 1985) reports, for anamorphous ribbon torque sensor, an output d.c. signal of 35 mV at anapplied torque of 10 N-M using an a.c. frequency of 20 kHz, an excitingcurrent of 120 mA, exciting coils having 220 turns each and sensingcoils having 80 turns each and a shaft diameter of 12 mm. Inasmuch assensitivity is directly proportional to a.c. frequency, exciting currentand number of turns on the exciting and sensing coils and inverselyproportional to the cube of the shaft diameter, the Sasada et alsensitivity corrected to an equivalent basis as that shown in FIG. 6hereof is 0.007 V/N-M. In other words, the torque transducer of thepresent invention is more than twice as sensitive as the amorphousribbon torque sensor of Sasada et al.

Although it might appear from the foregoing discussion of the absoluteand relative advantages of the nickel maraging steels that moreconventional steel alloys are not useful in accordance with the presentinvention, this is manifestly not the case. It is, of course, true thatall but the nickel maraging steels appear to suffer from the verysignificant drawback that thermal hardening, as by high temperatureheating followed by quenching to achieve through hardening or hightemperature heating in a carburizing atmosphere to achieve casehardening, thermally relieves the intentionally created residualstresses within the bands with the result that the large magnituderesidual stress-created magnetic anisotropies within the bands, whichare essential to the magnetoelastic torque transducers of the presentinvention, are in large part removed. However, it is clear from the datareported in FIG. 5 that each of the SAE 9310, 416SS and AISI 1018 alloystested exhibited a sensitivity, in terms of magnitude of output signalfor a given torque loading, within the useful range and each produced areasonably linear signal over most of the torque range. Thus, it is nottheir inherent magnetic characteristics which disqualify these alloysfrom the type of transducer use exemplified by the data reported in FIG.5, but rather that the thermal hardening process necessary to give analloy the mechanical and strength characteristics it needs to performits intended mechanical function appears to destroy the favorablemagnetic properties of these alloys. In this connection it w ill beappreciated that in most applications for the torque transducers of thepresent invention, the mechanical and strength characteristicsrequirements for the rotating shafts dictated by the operatingcharacteristics of the equipment in which they are installed necessitatethe use of steel alloys in their hardened condition.

In order to demonstrate this unfortunate adverse relationship betweenthe mechanical and magnetic properties of steel alloys, 1/8 inchdiameter shafts of 410 SS and 502 SS having the following compositionsin percent by weight were employed:

410 SS: 12.3 Cr; 0.2 Ni; 0.08 C; 0.9 Mn; 0.4 Si; 0.4 Mo; balance Fe

502 SS: 5.0 Cr; 0.06 C; 0.8 Mn; 0.4 Si; 0.5 Mo; balance Fe

The shafts were processed by annealing and torsionally overstraining, bytwisting both ends of a centrally restrained shaft, to provide a pair ofadjacent bands having respectively symmetrical right and left handhelically directed residual stress-created magnetic anisotropy.Thereafter, while still in their mechanically soft conditions, knowntorque loadings were applied to each processed shaft and the outputvoltage recorded. In each case the shafts displayed a good response tothe torque loadings, the 410 SS shaft exhibiting an output sensitivityof 600 mV over a torque loading range of approximately 2 newton-metersand good linearity. The 502 SS shaft was not quite as good, exhibiting,over the same torque loading range, an output sensitivity of only 95 mVand a less desirable linearity. Nevertheless, this rough test fortransducer function confirmed the FIG. 5 results that at least certainsteel alloys possess the potential for such function. When the testswere completed, the 410 SS and 502 SS shafts were subjected to ahardening heat treatment at about 950° C. followed by quenching. Whenthe torque loading tests were repeated on the hardened shafts it wasnoted that the output sensitivity over the same torque loading range haddropped to less than 5 mV, confirming, as expected, that the hardeningheat treatment had destroyed the desirable magnetic propertiesintentionally imparted to the shafts by the pre-testing processing.

In fact, notwithstanding the foregoing disability of steel alloys, ithas been determined that thermally hardenable steel alloys, as morefully and clearly defined hereinafter, can be made to function veryeffectively in the magnetoelastic torque transducers of the presentinvention. This can be accomplished with such alloys by creating theresidual stress within the bands only after the alloys have beenappropriately thermally treated to bring their hardness and strengthcharacteristics up to the levels required by the intended usage of thealloys. The subsequent or post-hardening creation of residual stress toendow the bands with the desired magnetic anisotropy does not adverselyalter the mechanical or strength properties of the alloys. However,preparing a steel alloy shaft in this manner does impose strictlimitations on the type of process which may be used to create theresidual stresses. Certain processes, like knurling which requiresplastic flow and torsional overtwisting, require a soft alloy and cannotbe practiced on already hardened alloys. Efforts to knurl hardened steelalloys has resulted in microcracks and fissures and other undesirabletopographic devastation to the alloy shaft. Efforts to twist smalldiameter hardened steel alloys has invariably resulted in snapping ofthe shafts. Thus, in accordance with the present invention, the use ofthermally hardenable non-Ni maraging steel alloys, as definedhereinafter, in the magnetoelastic torque transducers of the presentinvention is limited to creating the necessary residual stresses onlyafter the alloy has been mechanically strengthened by thermal treatmentand, then, only by selected residual stress creating processes which arecapable of endowing the bands with the desired corresponding andopposite uniform distribution of magnetic anisotropy. Such processesinclude, but are not limited to, grinding, mechanical scribing (withappropriate tools), directed or masked shot peening or sand blasting,chemical means and heat treatments, including laser scribing, induction,torch, thermal print head, and the like. As a result, although clearlynot as flexible in their usage as the nickel maraging steels which canbe residually stressed either before or after mechanical hardening andstrengthening, and thus are not process limited by the residual stresscreating technique employed, the non-Ni maraging steel alloys, ashereinafter defined, play an important role as the ferromagnetic,magnetostrictive shaft element in the magnetoelastic torque transducersof the present invention.

The selection of a residual stress creating process for creatingresidual stress within the bands of an already hardened steel alloy isbased upon many considerations, not the least of which are the size ofthe shaft, composition of the shaft, number of shafts to be processed,economics, including availability, of the necessary processing apparatusand final topographic appearance of the shaft. In many instances, as apractical matter, the latter consideration will be the controllingfactor. Manufacturers of rotating shaft equipment are likely to bereluctant to alter their equipment in any significant manner toaccomodate the installation of a magnetoelastic torque transducer,irrespective of the desirability of such an installation. More likely,at least until rotating shaft equipment is designed ab initio to includesuch torque transducers, is that a suitable transducer will have to bemade for each separate application. Considerations such as theenvironment in which the shaft rotates, the structural integrity of theshaft, the proximity of other components to the rotating shaft, and thelike, suggest that equipment manufacturers will prefer that the processemployed for forming the bands on or associated with their rotatingshafts be selected to minimize the macroscopic topographic alteration ofthe shaft surface.

In fact, apart from manufacturer reluctance, there are functionalreasons for minimizing macroscopic topographic alteration of the shaftsurface. In accordance with the present invention residual stress iscreated within the bands in order to endow the bands with intentionallyinstilled, controlled magnetic stress anistotropy of a relatively largemagnitude. One primary reason is to overwhelm and/or renderinsignificant uncontrolled and random magnetic anisotropies present inthe shaft in order that the torque transducers of the present inventionwill respond in a predictable manner to permeability changes caused byapplied torques. It is, therefore, desirable to minimize factorsaffecting these permeability changes other than the controlled factor ofresidual stress-created magnetic anisotropies intentionally instilled inaccordance with the present invention. In this connection it has beenfound that differential macroscopic topographic alteration of the shaftsurface such as knurls, slits, ridges, etc., affects the permeabilitychange detected by the magnetic discriminator in addition to the desiredmagnetoelastic effect. The non-magnetoelastic effect on permeabilitysensing of differential topographic alteration is threefold. There is amaterial induced, non-uniform, topographic effect as a result of whichthe shape and material symmetry in each band is differently altered ordistorted when the shaft is subjected to torque. Due to the resolutionof an applied torsional stress into orthogonal tensile and compressivestresses, the knurls, slits, etc. in one band will get longer and closertogether while the knurls, slits, etc. in the other band will getshorter and further apart. Exciting and sensing coils surrounding therespective bands will effectively see a different amount and/ordistribution of magnetic material in each band and, therefore, willsense a different permeability in each. In this manner there will be apermeability sensing effect due to the topographic alteration andindependent of the residual stress-created magnetic anisotropies in thebands. In addition, even in a totally annealed shaft, there is atopographic effect on the stress distributions resulting from theapplication of torque to the shaft. This topographic effect causes thetorsional stresses in the bands to distribute differently than theyotherwise would in a topographically unaltered band with the result thatat least a part of the permeability changes sensed by coils overlyingthe bands will be due to this topographically influenced stressdistribution and not solely due to the desired magnetoelastic effect.Finally, there is the effect on permeability sensing caused by the shapeof the topographic alteration. For example, assuming the bandsconsisted, respectively, of ±45° slits formed in the shaft surface, whena torsional stress is applied to the shaft and a cyclically time varyingmagnetic field is applied to the bands, depending upon whether thetorsional stress is applied CW or CCW, one of the tensile/compressivestresses tends to align the magnetization along the length of one set ofslits while the other tends to align the magnetization orthogonallythereto, i.e., in a direction perpendicular to the length of the otherset of slits. Apart from magnetoelastic considerations it is, as aphysical and configurational matter, easier for the appliedmagnetization to move along the former than the latter. As a result, thepermeability sensed by the coil overlying the band in which themagnetization seeks to align along the length of the slits is higherthan the permeability sensed by the coil overlying the band in which themagnetization seeks to align across the length of the slits. Thisincrease in permeability is due in part to the topographic alteration ofthe band surface. The magnetization is forced to follow a physicallymore difficult path in moving across rather than along the slits.

Cumulatively, these three reasons explain why all topographicallyaltered shafts work to some extent as torque transducers. It is also thereason why thermal treatment can never remove all permeability change ina topographically altered shaft. By the same token, the effect oftopographic alteration of the shaft surface is to introduce extraneous,undesirable, and uncontrolled signals into the permeability sensingfunction and, to this extent at least, to deviate from the idealsituation wherein strictly magnetoelastic considerations relate theapplied torque to the sensed output. Therefore, to the extent possible,macroscopic topographic alteration of the shaft surface should beavoided. This objective is entirely consistent with the application ofresidual stress after the shaft alloy has been suitably hardened andstrengthened by thermal treatment and, therefore, is entirely consistentwith the use of selected non-Ni maraging steel alloys, as hereinafterdefined, in connection with the present invention.

The various residual stress-creating techniques hereinbefore described,to greater or lesser extents, minimize macroscopic topographic surfacealteration. One readily demonstrable technique is grinding wherein aconventional grinding wheel inclined to the axis of the workpiece at±20° -60° in preparing the respective bands is used to remove an equallysmall depth of shaft surface, e.g., as little as 0.001 inch, dependingupon the geometry of the bands ,along the axial length of each band.Inasmuch as grinding merely removes an equal amount of alloy surfacematerial from the respective bands, there is no differential macroscopictopographic alteration--the change in each band is identical and thereare no non-uniform topographic features in each band. For all practicalpurposes, the two bands are topographically identical. A section takenperpendicular to the shaft axis in either band is a circle with onlymicroscopic deviations from a perfect circle. Yet the grinding techniquehas created two adjacent bands having opposite and correspondingmagnetic anisotropy due substantially completely to residual stresscreated by the grinding process. When torsional stress is applied to theshaft there is no material-induced, non-uniform topographic effect, suchas lengthening or shortening of ridges or troughs; there is notopographically-induced differential torsional stress distributionbecause the topographics of the bands are identical; there is no shapeeffect due to any topographic differential between the bands. Thisdiffers significantly from the situation which exists when knurledregions form the bands. Each knurled region contains differentarrangements of troughs and ridges, i.e., distortable shapes andfeatures, differential stress and magnetization influencingtopographics, and shape effect imparting features, which separately andcumulatively alter the sensed permeability independent of the residualstress created anisotropy intentionally instilled in the bands. Thus,the application of a torsional stress to a shaft containing spaced-apartknurled bands will always produce an output signal, but the signal willnot be solely a function of residual stress-created anisotropy--rather,it will be influenced by the macroscopic topographic alteration of theshaft surface represented by the knurl. As a consequence, all suchknurl-containing shafts will appear to function as torque transducers,although it may well be that their output signals are not at allinfluenced by residual stress-created magnetic anisotropies and bearonly a remote relationship to applied torque.

In order to demonstrate how magnetoelastic torque transducers inaccordance with the present invention may be made, using non-Ni maragingsteel alloys by the application of residual stress to create magneticanisotropy in the bands following hardening by thermal treatment,several different torque transducers were assembled using 1/4-inch and1/2-inch diameter cylindrical shafts having formed thereon by a grindingprocess a pair of axially spaced-apart bands endowed with helicallysymmetrical LH and RH magnetic easy axes. The characteristics of thisarrangement were sensed on the 1/2-inch shafts by positioning bobbinsconcentric with the shaft and axially aligned with the bands, eachbobbin having a magnetizing and sensing coil mounted thereon. Themagnetizing coils were connected in series and driven by an alternatingcurrent source. The emf induced in each of the sensing coils wasseparately rectified with the rectified outputs oppositely connected toproduce a difference signal which was displayed on a voltage displayinstrument. The characteristics of the arrangement were sensed on the1/4-inch diameter shafts using a conventional multivibrator circuit inwhich only one of two parallel transistors at a time conducts the inputfrom a high frequency source, thus allowing a square wave voltage tocreate a cyclically time varying magnetic field for application to thebands on the shaft with the result that the inductance of a coilassociated with one of the bands increases while the inductance of acoil associated with the other of the bands decrease. This difference ininductance produces different voltage signals which enter a comparatorfor producing a difference signal which was displayed on a voltagedisplay instrument.

Three shafts formed of different materials were employed for comparativepurposes. The materials were T-250, a nickel maraging steel hardened bymartensitizing and aging; AISI 416, a through hardened steel; and, SAE9310, a case hardened steel. The compositions of each alloy are setforth in percent by weight hereinbefore.

In a first series of runs, each of the alloys was ground in itsunhardened condition to form the desired bands of residualstress-created magnetic anisotropy. A known torque loading was appliedto each shaft under test and the output voltage signal was recorded. Theapplied torque was increased from zero up to as high as 30 newton-meters(N-M) with some shafts. FIG. 10, curve "1" (SAE 9310), FIG. 11, curve"3" (AISI 416) and FIG. 12, curve "5" (T-250) are graphs of appliedtorque versus output d.c. voltage for each shaft ground in theunhardened condition and tested. It is apparent that the sensitivityimparted to the shaft by grinding in each case was excellent and thelinearity of the output signal for each over the torque range tested wasgood, both observations confirming that grinding is an effectivetechnique for imparting residual stress-created magnetic anisotropy tothe bands formed on shafts of nickel maraging steel as well as non-Nimaraging steel alloys. Following testing, the nickel maraging steelshaft was aged at 480° C. for incremental periods from 20 minutes to 24hours. For each increment the performance of the shaft as a torquesensor appeared to improve. This result is consistent with the resultsobserved from FIGS. 5 and 6. The AISI 416 and SAE 9310 shafts werethermally treated at elevated temperatures of about 950° C. and 900° F.,respectively. When the foregoing torque loading tests were repeated, theshafts exhibited virtually no output sensitivity to applied torque. Thisresult is in accord with the previously observed results involvingtorsionally overstrained shafts. The high temperatures corresponding tothermal hardening heat treatment temperatures had destroyed thedesirable magnetic properties intentionally imparted to the shafts bythe grinding process. Moreover, inasmuch as virtually no outputsensitivity remained, it is clear that the grinding process producedinsignificant macroscopic topographic alteration of the shaft surfaces.

In a second series of runs, all of the alloys were hardened by thermaltreatments appropriate for hardening the particular alloy. The nickelmaraging steel shaft was aged at 480° C. for one hour; the AISI 416 wasannealed at 950° C. and quenched; and, the SAE 9310 was high temperaturetreated at about 900° C. in a carburizing atmosphere and quenched tocase harden the surface to a depth of 0.04 inches. Thereafter, each ofthe hardened alloy shafts was ground and tested in the same manner as inthe first series of runs. FIG. 10, curve "2" (SAE 9310), FIG. 11, curve"4" (AISI 416) and FIG. 12, curve "6" (T-250) are graphs of appliedtorque versus output d.c. voltage for each shaft ground in the hardenedcondition. It is interesting to note that the output sensitivity of thehardened-then-ground shafts was consistently lower than that of theunhardened ground shafts. Even with the nickel maraging steels, althoughadditional aging following grinding of the hardened-then-ground shaftsimproved the output sensitivity of the shaft, the maximum sensitivityattainable was still only about 2/3 of that attainable by grinding anunhardened nickel maraging steel shaft. However, thehardened-then-ground shafts consistently exhibited better linearity andhysteresis. These results confirmed that useful magnetoelastic torquetransducers can be made using appropriate steel alloys by a techniquewherein the residual stress-created bands of magnetic anisotropy areprovided to the shaft only after the shaft alloy has been suitablyhardened and strengthened by thermal treatment. The observed decrease inoutput sensitivity and increase in linearity and hysteresis isconsistent with the appreciation that in a hardened alloy shaft, whichhas a much higher elastic limit than a soft alloy shaft, the appliedgrinding stresses must be higher in order to exceed the elastic limitfor creating residual stress within the bands. The resulting residualstresses are, therefore, much higher than when grinding is accomplishedon a soft alloy shaft. As a result, the total quiescent anisotropy ("K")of the alloy shaft system (including anisotropy resulting from residualstresses, shape, crystallinity, etc.) before the application oftorsional stress to the shaft is higher for a hardened than for a softalloy shaft. It will be appreciated that the permeability effect of anapplied stress is directly proportional to the magnetostriction of thealloy and the applied stress and is inversely proportional to thequiescent anisotropy of the system. Therefore, when torque is applied toa high "K" system, the effect of the applied stress is smaller and theobserved sensitivity or effect on permeability is likewise smaller. Forsubstantially the same reasons, the higher "K" causes a lower hysteresisand improved linearity.

It has already been noted that effective magnetoelastic torquetransducers in accordance with the present invention can be made usingnickel maraging steel as the ferromagnetic and magnetostrictive shaftcomponent and instilling desired magnetic anisotropy in the shaft bycreating residual stress within the bands either before or after thenickel maraging steel alloy has been aged to improve its mechanical andstrength properties. It has also been noted that effectivemagnetoelastic torque transducers in accordance with the presentinvention can be made using selected non-Ni maraging steel alloys andcreating the residual stress within the bands only after the non-Nimaraging steel alloys have been appropriately thermally treated toimprove their mechanical and strength characteristics. It is clear formany reasons that not all non-Ni maraging steel alloys can be soemployed. Typically, in annealed iron-carbon alloys of varying carboncontents, the magnetostriction, i.e., the functional change in length inan applied magnetic field, is anisotropic and, in some directions, isnegative. In many instances the alloy undergoes a Villari reversal --itfirst expands with increasing field strength, i.e., the fractionalchange in length is positive, and then contracts, i.e., the fractionalchange in length is negative with increasing field strength. When torqueis applied to a shaft formed of an alloy which undergoes such a reversalthe stress-applied anisotropy affects the existing random anisotropydifferently in shaft areas of positive magnetostriction than in areas ofnegative magnetostriction. This leads to a varying combined anisotropyat different locations in the shaft due to the anisotropic localmagnetostriction with the result that a uniform, resulting magnetizationcannot be obtained. For obvious reasons this is an unacceptablecondition in a torque transducer and alloys which undergo the Villarireversal are obviously unacceptable for use. It has been known for sometime that alloying with certain elements, e.g., nickel, causes themagnetostriction of the resultant alloy to become more positive. Infact, it has been noted that only 1 to 3 percent by weight nickel isneeded to eliminate the Villari reversal contraction of iron-carbonalloys. Thus, as the original iron-carbon alloy becomes richer in nickelthe alloy behaves as though its magnetostriction is positive at allfield strengths such that the combined anisotropy at different locationsin the shaft becomes more uniform and a uniform resulting magnetizationcan be obtained. A similar trend has been noted with the addition ofother alloying elements, e.g., chromium (Cr), cobalt (Co), titanium(Ti), aluminum (Al), manganese (Mn), molybdenum (M o), copper (Cu),boron (B), and combinations thereof, toward making the magnetostrictionof the iron-carbon alloy system more positive. Another means for dealingwith the Villari reversal in the iron-carbon alloy system is to annealat high temperatures of about 750-800° C. and then quench withoutfurther annealing to temper. Such a treatment appears to substantiallyeliminate the Villari reversal to produce a hardened iron-carbon alloysystem having a substantially isotropic magnetostriction. Hardening bythermal treatment, in addition, eliminates cold working stresses and theattendant random anisotropy due to prior processing and improves thehysteresis and linearity response aspects of the effect of torsionalstress on permeability.

It has, therefore been determined, in accordance with the presentinvention, that non-Ni maraging steel alloys useful as theferromagnetic, magnetostrictive region of or associated with therotating shaft should have a substantially isotropic, highmagnetostriction having an absolute value of at least 5 ppm; beenhardened by thermal treatment; a carbon content, in percent by weight,in the range from 0.05 to 0.75 percent; an alloy composition tailored tothe mechanical, chemical and magnetic (resulting anisotropy andmagnetostriction) requirements of the intended application of the shaft.

All of the foregoing can be achieved by selecting the non-Ni maragingsteel alloy useful in the present invention from thermally hardenedsteel alloys characterized by an isotropic magnetostriction having anabsolute value of at least 5 ppm and a composition comprising thefollowing elements in the indicated proportions:

    FeC.sub.a M.sub.b Q.sub.c

wherein "M" is one or more alloying elements selected from the groupconsisting of Ni, Cr, Co, Ti, Al, Mn, Mo, Cu and B,

"Q" is one or more other alloying elements including, but not limitedto, common steel alloying elements such as silicon (Si), phosphorous(P), sulfur (S), nitrogen (N), selenium (Se), tungsten (W), vanadium(V), hafnium (Hf), columbium (Cb), tantalum (Ta) and tin (Sn);

"a" indicates a carbon content of from 0.05 to 0.75 percent by weight;

"b" indicates a content of alloying element(s) "M" at least sufficientto raise the magnetostriction of the alloy to the at least 5 ppmabsolute level, the desired level of magnetostriction depending upon thequiescent anisotropy ("K") and applied stress due to applied torque forany given application;

"c" indicates a content of alloying element,(s) "Q" from zero up to anyuseful quantity dependent upon the desired mechanical, chemical, and/orother properties of the alloy.

Applying the foregoing definition of a suitable non-Ni maraging steelalloy to several of the illustrative steel alloys discussed hereinbeforeand applying published magnetostriction data for various of the alloyingelements, the magnetostriction of the alloys can be at least estimated.For example: for AISI 410, the presence of 12.3% Cr is primarilyresponsible for raising the magnetostriction of the alloy to about 20ppm; for AISI 502, the presence of 5% Cr and 0.8 Mn are primarilyresponsible for raising the magnetostriction of the alloy to about 7ppm; for SAE 9310, the presence of 3-3.5% Ni and 1-1.4% Cr are primarilyresponsible for raising the magnetostriction of the alloy to about 15ppm.

Industrial Applicability

The unique and improved magnetoelastic torque transducers of the presentinvention are broadly useful for the sensing and measurement of torquein members of all types and sizes, whatever may be the device or fieldof application in which the member operates. It is universally acceptedthat torque is an absolutely fundamental parameter in the control ofsystems having rotating members. Sensing the instantaneous torqueexperienced by a rotating member and generating an electrical current inresponse thereto which bears a known relationship to the torque allowsthe early diagnosis of incipient problems or the control, viamicroprocessor or otherwise, of the engine, machine, motor, etc. whichdrives the rotary member.

Applications for the torque transducers of the present invention can befound in virtually every device having a rotating member. There alreadyis a demand for sensitive, responsive, and inexpensive magnetic torquesensors for monitoring torque in engines and power drives to improveoverall performance and fuel economy, control exhaust emissions andmodulate transmission ratios; in marine propulsion systems to detect andcorrect reduced output from the propulsion machinery and the effects ofhull fouling and propeller damage; in helicopter turbines to avoidoverloading and to detect power loss caused, for example, by sand orsalt spray. There is also a demand for torque transducers such as areprovided in accordance with the present invention for controlling heavyindustrial machinery of all types, e.g., pulp grinders for maintainingfiber quality, paper-making machines, and the like, as well as for usein consumer home and commercial appliances, e.g., food mixers andprocessors. In addition to their use in internal combustion engines,power transmission means, fluid turbine means, etc., the need for small,inexpensive, sensitive, reliable torque sensors has been noted in suchdiverse applications as machine tools, hand tools, robotics, forcemeasuring systems, information devices, industrial measuringinstruments, weighing systems of various kinds, electronic powerassisted power steering, and vehicular traction balancing.

FIGS. 13-18 schematically illustrate several of these applications.Referring, for example, to FIG. 13, an internal combustion engine 200 orpower transmission means 202 or fluid turbine means 204 includes atorque-carrying output member 206, the member further including amagnetoelastic torque transducer 208, as described and illustratedherein, for sensing the torque developed in member 206. In FIG. 14,there is illustrated a weighing system 210 including torque-carryingmeans 212 and a magnetoelastic torque transducer 214, as described andillustrated herein, for sensing the torque developed in means 212. FIG.15 shows a machine tool 216 including means 218 for causing relativerotation between a tool 220 and a workpiece 222, the machine tool havinga torque-carrying member 224 including a magnetoelastic torquetransducer 226, as described and illustrated herein, for sensing thetorque developed in member 224. A robotic device 228 including amagnetoelastic torque transducer 230, as described and illustratedherein, is illustrated in FIG. 16. The robotic device comprisesmechanical means 232 for performing work pursuant to pre-programmed orreal time control instructions and a torque-carrying member 234. In FIG.17, there is depicted a vehicular steering system 236 havingtorque-carrying shaft means 238, the shaft means including amagnetoelastic torque transducer 240, as described and illustratedherein, for sensing the torque developed in shaft means 238. FIG. 18shows a force measuring system 242 including means 244 for converting asensed force, F, to torque and a magnetoelastic torque transducer 246,as described and illustrated herein, for sensing the torque resultingfrom the conversion by means 244.

One application for the magnetoelastic torque transducers of the presentinvention which is particularly promising in view of the potentialcontribution of these transducers to energy conservation, environmentalcleanliness and safety and because it directly affects so many peopleand businesses is its use on internal combustion engines and associatedengine power drives. The torque sensor of the present invention iscapable of recovering the torque signature of an engine over a wideenough bandwidth to discern salient details of important torquecontributing events at all points between idle and the top operatingspeed of the engine. Torque sensing in an accurate and cost effectivemanner enables early diagnosis of incipient problems due to thefunctional condition of the engine, helps to avoid unanticipatedfailures that might limit the servicability of the vehicle at criticaltimes and improves and/or controls the performance and economy of theengine and its power drive.

Primary power for the propulsion and other essential functions of modernvehicles is obtained from the rotating output shaft of an internalcombustion engine. Regardless of the type of engine the power actuallydelivered by this shaft to the vehicle is the numerical product of onlytwo parameters: rotational speed and transmitted torque. Of the two,torque is the intensive parameter since rotational speed is itselfconsequential to the internally developed torque of the engine. It isthe magnitude of available torque that sets the limits on vehicleacceleration, its speed on grade and other mobility and performancefactors. The successful use and enjoyment of the vehicle depends,ultimately, on the ability of its engine to deliver the functionallyrequired torque throughout its operational range of speeds.

Except for the situation where a turbine engine is driving a constantload, the torque transmitted through an engine output shaft fluctuatesrapidly. These fluctuations reflect both the cyclic variations in thetorque developed by the engine and transient variations in the torqueimposed by vehicle loads. In piston engines, torque is developed by eachcylinder only during its power stroke. Multicylinder engines attain somecontinuity of developed torque by the overlap of phased power strokesfrom each cylinder. While cyclic variations in output torque are alsoreduced thereby, and further reduced by the combined inertia of theengine's internal moving parts, the strongly impulsive nature of eachcylinder's developed torque is still transmitted through the outputshaft. Cyclically stimulated torsional vibrations together with thechanging accelerations of linked reciprocating parts contributeadditional time varying torque components. The magnitude and even thedirectional sense of this torque is further influenced by variations inoperational conditions of the vehicle, e.g., throttle settings, gearpositions, load pick-up, road surface inclination and roughnessfeatures.

Although the torque on the engine output shaft represents thesuperposition of contributions from this multiplicity of sources, manyare strongly interdependent and their combination forms an effectivesignature characterizing the engine's performance. Salient features ofthis signature would clearly correlate with specific engine events,e.g., cylinder firings. The absence of a normal feature, its alterationor the development of new features would be a reflection of adysfunction. The nature and extent of the abnormality would besymptomatic of specific engine or drive line difficulties. While manyengine problems are also detectable by their symptomatic effects onoverall performance and/or more objectively measurable quantities (e.g.,manifold pressure, compression, noise signature, exhaust gas analysis),none are as sensitively quantified as torque to the individual eventswhich together characterize proper engine function. Since torque is theeffective product of the engine, no measurements of indirectly relatedparameters can so clearly identify the source of inadequate productionas can the measurement of torque itself. Conventional methods ofrecovering torque data, whether by dynamometer or from measurements ofunloaded engine acceleration and deceleration by procedures involvingstepped changes in fuel flow and/or ignition interruption, determineonly average values and lack the detail needed for clear diagnosis andcontrol. Recovery and analysis of the information contained in thetorque signature of the engine output shaft enables diagnosis ofincipient problems, helps to avoid unanticipated failures that mightlimit the servicability of the vehicle at critical times and improvesand/or controls the performance and economy of the engine and its powerdrive. The key to the problem is the recovery of enough torqueinformation for a meaningful analysis.

In a 12 cylinder, 4 stroke engine operating at 4000 rpm there are 400power strokes and (at least) 1600 valving events (openings or closings)every second. Turbine engines run with far smoother power input but atspeeds up to 500 revolutions per second. To be capable of discriminatingimportant details of these salient events, the torque sensing systemmust have a reasonably flat frequency response up to at least severaltimes the maximum event rate, i.e., in the vicinity of 5 kHz. Thefrequency response must also extend downward to zero Hz to faithfullycapture the steady state torque components imposed by the vehicle loads.

Although that full bandwidth is obviously desirable for maximum utilityas a diagnostic tool, the information contained in the low frequencyspectrum, up to 10 Hz, accurately describes the engine's overallresponse to control (input) and load (output) changes. Not only canvariations in performance be objectively evaluated from this informationbut i t also has potentially prime utility in another area, control ofthe engine and associated power drive.

A torque sensor having 5 kHz bandwidth capability cannot be positionedarbitrarily. While torque is applied to the engine shaft by contactforces at discrete locations, it is transmitted axially by continuousstress distributions. Transient torque events are not transmittedinstantaneously nor do they remain unaltered along the shaft. The finiteelasticity and inertia of real shaft materials combine to limit thetransmittable rate of change of torque. Steep transients triggeroscillatory exchanges of elastic and kinetic energy (stress waves) whichtravel with material and mode dependent characteristic velocities alongthe shaft. The fidelity of the transmitted torque is further reducedwith distance from its source by the accumulated dissipative effects ofinternal and external friction. The sensor must therefore be locatedclose enough to the source(s) to avoid losing the desired torqueinformation either by attenuation or in background "noise" composed ofcomplex combinations of interfering and reflecting stress waves.

Important sensor requirements are that it be small, at least in thedimension parallel to the shaft axis, that it be rugged and that it befree from deteriorating effects of use or time such as wear, corrosionor fatigue. The sensor should be amenable to performance verificationand calibration, especially in the event of repair or replacement ofparts of the torque sensing system, including the engine shaft. Itshould have neglible impact on engine and drive line manufacturability,operation and maintenance and, under no circumstances should the failureof the torque sensor have any contingent consequences which interferewith the otherwise normal operation of the vehicle.

The context is clear, whether for engines, power drives or other uses, asuitable torque sensor should be an unobtrusive device that is difficultto abuse and is capable of reliably recovering much of the torqueinformation available on the torqued shaft. None of the heretoforecontemplated state of the art torque transducers can meet theserequirements. However, the magnetoelastic torque sensors of the presentinvention appear eminently suitable in all respects and will, for thefirst time, make inexpensive, reliable and sensitive torque sensorsavailable for commercial implementation.

I claim:
 1. A magnetoelastic torque transducer for providing anelectrical signal indicative of the torque applied to a member, saidmember having a ferromagnetic and magnetostrictive region formed of athermally hardened steel alloy characterized by a substantiallyisotropic magnetostriction having an absolute value of at least 5 ppmand including from 0.05 to 0.75 percent by weight carbon and sufficientof an element selected from the group consisting of nickel, chromium,cobalt, titanium, aluminum, manganese, molybdenum, copper, boron, andcombinations thereof to raise said alloy magnetostriction value to saidat least 5 ppm absolute, said transducer comprising:a pair of axiallyspaced-apart annular bands defined within said region, said bands havingrespectively symmetrical right and left hand helically directed residualstress created magnetic anisotropy of sufficiently large magnitudecompared with the random magnetic anisotropy in said member that thecontribution to total magnetic anisotropy of any random anisotropy isnegligible, each said band having at least one circumferential regionwhich is free of residually unstressed areas over at least 50% of itscircumferential length; means for applying a cyclically time varyingmagnetic field to said bands; means for sensing the change inpermeability of said bands caused by said applied torque; and, means forconverting said sensed change in permeability to an electrical signalindicative of the magnitude of the torque applied to said member.
 2. Amagnetoelastic torque transducer, as claimed in claim 1, wherein eachsaid band has at least one circumferential region which is free ofresidually unstressed areas over at least 80% of its circumferentiallength.
 3. A magnetoelastic torque transducer, as claimed in claim 1,wherein said band has at least one continuous circumferential regionwhich is free of residually unstressed areas.
 4. A magnetoelastic torquetransducer, as claimed in claim 1, wherein said alloy further includesfrom zero up to a useful quantity for imparting desired properties tosaid alloy of an element selected from the group consisting of silicon,phosphorous, sulfur, nitrogen, selenium, tungsten, vanadium, hafnium,columbium, tantalum, tin, and combinations thereof.
 5. A magnetoelastictorque transducer, as claimed in claim 1, wherein said region is formedof a thermally hardened steel alloy consisting essentially of theelements and proportions indicated by the general formula:

    FeC.sub.a M.sub.b Q.sub.c

wherein: "M" is an element selected from the group consisting of nickel,chromium, cobalt, titanium, aluminum, manganese, molybdenum, copper,boron, and combinations thereof; "Q" is an element selected from thegroup consisting of silicon, phosphorous, sulfur, nitrogen, selenium,tungsten, vandadium, hafnium, columbium, tantalum, tin and combinationsthereof; "a" indicates a carbon content of from 0.05 to 0.75 percent byweight; "b" indicates a content of element "M" at least sufficient toraise the magnetostriction of said alloy to at least 5 ppm absolute; and"c" indicates a content of element "Q" from zero to a useful quantityfor imparting desired properties to said alloy.
 6. A magnetoelastictorque transducer, as claimed in claim 1, wherein said region forms apart of the surface of said member.
 7. A magnetoelastic torquetransducer, as claimed in claim 1, wherein said region is rigidlyaffixed to the surface of said member.
 8. A magnetoelastic torquetransducer, as claimed in claim 1, wherein said region is formed of asteel alloy selected from the group consisting of through hardenable andcase hardenable steel alloys.
 9. A magnetoelastic torque transducer, asclaimed in claim 1, wherein the magnetic easy axes in said bands areoriented, respectively, at angles of ±20° -60° to the axis of saidmember.
 10. A magnetoelastic torque transducer, as claimed in claim 1,wherein said region is formed of nickel maraging steel and said bandsare defined and said residual stress-created magnetic anisotropy hasbeen instilled within a mechanically soft portion of said region.
 11. Amagnetoelastic torque transducer for providing an electrical signalindicative of the torque applied to a member, said member having aferromagnetic and magnetostrictive region, said transducer comprising:apair of axially spaced-apart annular bands defined within a thermallyhardened portion of said region, said bands having respectivelysymmetrical right and left hand helically directed residual stresscreated magnetic anisotropy instilled therein subsequent to thermalhardening of sufficiently large magnitude compared with the randommagnetic anisotropy in said member that the contribution to totalmagnetic anisotropy of any random anisotropy is negligible, each saidband having at least one circumferential region which is free ofresidually unstressed areas over at least 50% of its circumferentiallength; means for applying a cyclically time varying magnetic field tosaid bands; means for sensing the change in permeability of said bandscaused by said applied torque; and means for converting said sensedchange in permeability to an electrical signal indicative of themagnitude of the torque applied to said member.
 12. A magnetoelastictorque transducer, as claimed in claim 11, wherein said region is formedof a steel alloy selected from the group consisting of iron-nickelmartensite hardenable and thermally hardened steel alloys characterizedby a substantially isotropic magnetostriction having an absolute valueof at least 5 ppm and including from 0.05 to 0.75 percent by weightcarbon and sufficient of an element selected from the group consistingof nickel, chromium, cobalt, titanium, aluminum, manganese, molybdenum,copper, boron, and combinations thereof to raise said alloymagnetostriction value to said at least 5 ppm absolute.
 13. In amagnetoelastic torque transducer for providing an electrical signalindicative of the torque applied to a member including ferromagnetic,magnetostrictive means associated with said member for altering inmagnetic permeability in response to the application of torque to saidmember, means for applying a magnetic field to said ferromagneticmagnetostrictive means, means for sensing the change in permeabilitycaused by said applied torque, and means for converting said sensedchange in permeability to an electrical signal indicative of themagnitude of the torque applied to said member, the improvementcomprising forming said ferromagnetic, magnetostrictive means from athermally hardened steel alloy characterized by a substantiallyisotropic magnetostriction having an absolute value of at least 5 ppmand including from 0.05 to 0.75 percent by weight carbon and sufficientof an element selected from the group consisting of nickel, chromium,cobalt, titanium, aluminum, manganese, molybdenum, copper, boron, andcombinations thereof to raise said alloy magnetostriction value to saidat least 5 ppm absolute.
 14. A magnetoelastic torque transducer, asclaimed in claim 13, wherein said alloy further includes from zero up toa useful quantity for imparting desired properties to said alloy of anelement selected from the group consisting of silicon, phosphorous,sulfur, nitrogen, selenium, tungsten, vanadium, hafnium, columbium,tantalum, tin, and combinations thereof.
 15. A magnetoelastic torquetransducer, as claimed in claim 13, wherein said means is formed of athermally hardened steel alloy consisting essentially of the elementsand proportions indicated by the general formula:

    FeC.sub.a M.sub.b Q.sub.c FeC

wherein: "M" is an element selected from the group consisting of nickel,chromium, cobalt, titanium, aluminum, manganese, molybdenum, copper,boron, and combinations thereof; "Q" is an element selected from thegroup consisting of silicon, phosphorous, sulfur, nitrogen, selenium,tungsten, vandadium, hafnium, columbium, tantalum, tin and combinationsthereof; "a" indicates a carbon content of from 0.05 to 0.75 percent byweight; "b" indicates a content of element "M" at least sufficient toraise the magnetostriction of said alloy to at least 5 ppm absolute; and"c" indicates a content of element "Q" from zero to a useful quantityfor imparting desired properties to said alloy.
 16. A magnetoelastictorque transducer, as claimed in claim 13, wherein said ferromagnetic,magnetostrictive means forms a part of the surface of said member.
 17. Amagnetoelastic torque transducer, as claimed in claim 13, wherein saidferromagnetic, magnetostrictive means is rigidly affixed to the surfaceof said member.
 18. A magnetoelastic torque transducer, as claimed inclaims 13, 16, or 17, wherein said ferromagnetic, magnetostrictive meansis formed of a steel alloy selected from the group consisting of throughhardenable and case hardenable steel alloys.
 19. A magnetoelastic torquetransducer, as claimed in claims 13, 16, or 17, wherein at least aportion of said ferromagnetic, magnetostrictive means is endowed withhelically directed residual stress created magnetic anisotropy, at leastone circumferential region of said portion being free of residuallyunstressed areas over at least 50% of its circumferential length, saidapplying means applying said magnetic field to said endowed portion andto an area of said member not so endowed, said sensing means sensing thepermeability difference between said portion and said area resultingfrom the application of torque to said member, said converting meansconverting said sensed permeability difference to an electrical signalindicative of the magnitude of the applied torque.
 20. A magnetoelastictorque transducer, as claimed in claim 19, wherein said portion is athermally hardened portion of said ferromagnetic, magnetostrictivemeans, said residual stress-created magnetic anisotropy having beeninstilled in said portion subsequent to thermal hardening.
 21. Amagnetoelastic torque transducer, as claimed in claim 19, wherein saidcircumferential region is free of residually unstressed areas over atleast 80% of its circumferential length.
 22. A magnetoelastic torquetransducer, as claimed in claim 19, wherein said portion has at leastone continuous circumferential region which is free of residuallyunstressed areas.
 23. A magnetoelastic torque transducer, as claimed inclaims 13, 16, or 17, wherein said ferromagnetic, magnetostrictive meansincludes a pair of axially spaced-apart annular bands definedtherewithin, said bands having respectively symmetrical right and lefthand helically directed residual stress created magnetic anisotropy,each said band having at least one circumferential region which is freeof residually unstressed areas over at least 50% of its circumferentiallength, said applying means applying said magnetic field to said bands,said sensing means sensing the change in permeability of said bandscaused by said applied torque.
 24. A magnetoelastic torque transducer,as claimed in claim 23, wherein each said band has at least onecircumferential region which is free of residually unstressed areas overat least 80% of its circumferential length.
 25. A magnetoelastic torquetransducer, as claimed in claim 23, wherein each said band has at leastone continuous circumferential region which is free of residuallyunstressed areas.
 26. A magnetoelastic torque transducer, as claimed inclaim 23, wherein the magnetic easy axes in said bands are oriented,respectively, at angles of ±20° -60° to the axis of said member.
 27. Amagnetoelastic torque transducer, as claimed in claim 23, wherein saidbands are defined within a thermally hardened portion of said means,said residual stress-created magnetic anisotropy having been instilledin said bands subsequent to thermal hardening.
 28. A magnetoelastictorque transducer, as claimed in claim 23, wherein said means is formedof a thermally hardened steel alloy consisting essentially of theelements and proportions indicated by the general formula:

    FeC.sub.a M.sub.b Q.sub.c

wherein: "M" is an element selected from the group consisting of nickel,chromium, cobalt, titanium, aluminum, manganese, molybdenum, copper,boron, and combinations thereof; "Q" is an element selected from thegroup consisting of silicon, phosphorous, sulfur, nitrogen, selenium,tungsten, vandadium, hafnium, columbium, tantalum, tin and combinationsthereof; "a" indicates a carbon content of from 0.05 to 0.75 percent byweight; "b" indicates a content of element "M" at least sufficient toraise the magnetostriction of said alloy to at least 5 ppm absolute; and"c" indicates a content of element "Q" from zero to a useful quantityfor imparting desired properties to said alloy.
 29. In a method ofsensing the torque applied to a member having ferromagnetic,magnetostrictive means associated therewith which includes the steps ofapplying a magnetic field to said ferromagnetic, magnetostrictive means,sensing the change in permeability caused by said applied torque andconverting said sensed change in permeability to an electrical signalindicative of the magnitude of the applied torque, the improvementcomprising forming said ferromagnetic, magnetostrictive means of athermally hardened steel alloy characterized by a substantiallyisotropic magnetostriction having an absolute value of at least 5 ppmand including from 0.05 to 0.75 percent by weight carbon and sufficientof an element selected from the group consisting of nickel, chromium,cobalt, titanium, aluminum, manganese, molybdenum, copper, boron, andcombinations thereof to raise said alloy magnetostriction value to saidat least 5 ppm absolute.
 30. A method, as claimed in claim 29, whereinsaid alloy further includes from zero up to a useful quantity forimparting desired properties to said alloy of an element selected fromthe group consisting of silicon, phosphorous, sulfur, nitrogen,selenium, tungsten, vanadium, hafnium, columbium, tantalum, tin, andcombinations thereof.
 31. A method, as claimed in claim 29, wherein saidsteel alloy is selected from the group consisting of through hardenableand case hardenable steel alloys.
 32. A method, as claimed in claim 29,wherein said ferromagnetic, magnetostrictive means is endowed with apair of axially spaced-apart annular bands having respectivelysymmetrical right and left hand helically directed residualstress-created magnetic anisotropy, the permeability difference betweenthe bands is sensed, and said sensed permeability difference isconverted to an electrical signal indicative of the magnitude of theapplied torque.
 33. A method, as claimed in claim 32, wherein saidferromagnetic, magnetostrictive means is thermally hardened, at leastwithin the region thereof wherein said bands are located, for impartingdesirable mechanical properties to the material from which said regionis formed and endowing said thermally hardened means with said bandshaving said residual stress-created anisotropy.
 34. In a method ofsensing the torque applied to a member having a ferromagnetic andmagnetostrictive region including the steps of endowing at least aportion of said region with helically directed residual stress-createdmagnetic anisotropy, applying a cyclically time varying magnetic fieldto said portion and to an area of said member not so endowed, andsensing the permeability difference between said portion and said arearesulting from the application of torque to said member, the differencebeing indicative of the magnitude of the applied torque, the improvementcomprising forming said region of a thermally hardened steel alloycharacterized by a substantially isotropic magnetostriction having anabsolute value of at least 5 ppm and including from 0.05 to 0.75 percentby weight carbon and sufficient of an element selected from the groupconsisting of nickel, chromium, cobalt, titanium, aluminum, manganese,molybdenum, copper, boron, and combinations thereof to raise said alloymagnetostriction value to said at least 5 ppm absolute.
 35. A method asclaimed in claim 34, wherein said alloy further includes from zero up toa useful quantity for imparting desired properties to said alloy of anelement selected from the group consisting of silicon, phosphorous,sulfur, nitrogen, selenium, tungsten, vanadium, hafnium, columbium,tantalum, tin, and combinations thereof.
 36. A method as claimed inclaim 34, wherein said portion is thermally hardened, at least wheresaid portion is endowed with said magnetic anisotropy, for impartingdesirable mechanical properties to the material from which said regionis formed, and endowing said thermally hardened portion with saidmagnetic anisotropy.
 37. In a method of sensing the torque applied to amember having a ferromagnetic and magnetostrictive region, including thesteps of endowing a pair of axially spaced-apart annular bands withinsaid region with respectively symmetrical right and left hand helicallydirected magnetic anisotropy, applying a cyclically time varyingmagnetic field to said bands, and sensing the permeability differencebetween said bands resulting from the application of torque to saidmember, the difference being indicative of the magnitude of the appliedtorque, the improvement comprising:thermally hardening said member, atleast within said region, for imparting desirable mechanical propertiesto the material from which said region is formed; defining said bands atthe surface of said thermally hardened member; and, endowing said bandswith magnetic anisotropy by instilling a residual stress distribution ineach band which is sufficiently extensive that at least onecircumferential region within each band is free of residually unstressedareas over at least 50% of its circumferential length.
 38. A method, asclaimed in claim 37, wherein said instilled residual stress distributionis sufficiently extensive that said region is free of residuallyunstressed areas over at least 80% of its circumferential length.
 39. Amethod, as claimed in claim 37, wherein said instilled residual stressdistribution is sufficiently extensive that each said band has at leastone continuous circumferential region which is free of residuallyunstressed areas.
 40. A method, as claimed in claims 37, 38 or 39,wherein said region is formed of a steel alloy selected from the groupconsisting of iron-nickel martensite hardenable and thermally hardenedsteel alloys characterized by a substantially isotropic magnetostrictionhaving an absolute value of at least 5 ppm and including from 0.05 to0.75 percent by weight carbon and sufficient of an element selected fromthe group consisting of nickel, chromium, cobalt, titanium, aluminum,manganese, molybdenum, copper, boron, and combinations thereof to raisesaid alloy magnetostriction value to said at least 5 ppm absolute.
 41. Amethod as claimed in claim 40, wherein said alloy further includes fromzero up to a useful quantity for imparting desired properties to saidalloy of an element selected from the group consisting of silicon,phosphorous, sulfur, nitrogen, selenium, tungsten, vanadium, hafnium,columbium, tantalum, tin, and combinations thereof.
 42. A method, asclaimed in claims 37, 38 or 39, wherein said region is formed of athermally hardened steel alloy consisting essentially of the elementsand proportions indicated by the general formula:

    FeC.sub.a M.sub.b Q.sub.c

wherein: "M" is an element selected from the group consisting of nickel,chromium, cobalt, titanium, aluminum, manganese, molybdenum, copper,boron, and combinations thereof; "Q" is an element selected from thegroup consisting of silicon, phosphorous, sulfur, nitrogen, selenium,tungsten, vandadium, hafnium, columbium, tantalum, tin and combinationsthereof; "a" indicates a carbon content of from 0.05 to 0.75 percent byweight; "b" indicates a content of element "M" at least sufficient toraise the magnetostriction of said alloy to at least 5 ppm absolute; and"c" indicates a content of element "Q" from zero to a useful quantityfor imparting desired properties to said alloy.
 43. An internalcombustion engine having a torque-carrying output member, said memberincluding a magnetoelastic torque transducer as claimed in claims 1 or13.
 44. Power transmission means having a torque-carrying output member,said member including a magnetoelastic torque transducer as claimed inclaims 1 or
 13. 45. Fluid turbine means having a torque-carrying outputmember, said member including a magnetoelastic torque transducer asclaimed in claims 1 or
 13. 46. A weighing system includingtorque-carrying means, said means including a magnetoelastic torquetransducer as claimed in claims 1 or
 13. 47. A machine tool includingmeans for causing relative rotation between a tool and a workpiece, saidmachine tool having a torque-carrying member, said member including amagnetoelastic torque transducer as claimed in claims 1 or
 13. 48. Arobotic device comprising mechanical means for performing work pursuantto pre-programmed or real time control instructions, said device havinga torque-carrying member, said member including a magnetoelastic torquetransducer as claimed in claims 1 or
 13. 49. A vehicular steering systemhaving torque-carrying shaft means, said means including amagnetoelastic torque transducer as claimed in claims 1 or
 13. 50. Aforce measuring system including means for converting a sensed force totorque, a torque transducer connected for sensing said torque, saidtorque transducer comprising a magnetoelastic torque transducer asclaimed in claims 1 or 13.